Once your landings have been made, and bridgeheads consolidated so that you can reinforce as necessary, you will be able to begin expanding out from these footholds. You will now be looking at moving around through the Earth’s different environmental conditions, and seeking out further objectives to be secured.
Whatever your ultimate aim for the planet, you will now have to be able to work and fight on the surface, in the sea, and in the atmosphere. This means you will require vehicles and equipment, and will need to be prepared for a wide variety of climates and varying conditions in the zones of different types of operations to be conducted.
You will also need to be aware of various environmental situations, and not just those related to active resistance by the natives. It’s a jungle out there – and desert, ocean, glacier, veldt… You must be prepared to live, thrive, and survive in all of these different types of environment if you are to seal your conquest of the planet.
Before discussing vehicles, you will need to be sure that your forces can withstand the terrestrial environment when outside of any vehicles. You may need pressure suits, if you evolved in a world with a thinner or denser atmosphere, and breathing equipment if the Earth’s atmosphere is not conducive to supporting your form of life.
If you have come from a world with a lower gravity, or are adapted to life in microgravity aboard space vessels, you will probably require support structures for your physical form – limbs and spine, if you have them, for example – in order to withstand long periods on the surface of the Earth. Conversely, if you come from a higher gravity planet, you may find that there are degenerative side effects to being in a lower gravity environment for a long time, though you will probably have already discovered and addressed this issue while becoming adjusted to space travel. (But, see also the chapter on ‘Looking After Your Humans’.)
Even if you have encountered the issues of physical degeneration in microgravity, do not assume that these factors cannot still affect you on the surface of the Earth, if you originally came from a location with higher gravity. In such an instance, you may be tempted to assume you will be physically stronger than the natives of a lower gravity planet, but this is not necessarily the case, if your physiology is adapted for load-bearing rather than leverage.
Most of the inhabited regions are temperate in nature, neither too hot nor too cold, though even these areas can reach temperatures over between -25C and 35C at the extremes of the seasons.
Humidity, the amount of water vapour suspended in the air, varies wildly according to location, weather, time of year, type and amount of vegetation, topography, and a host of other factors. In general, areas with large amounts of trees and rainfall will have high humidity, especially in summer. This is particularly the case in regions around the equator, bounded by the tropics at 23 degrees north and south of the equator. This belt is a side effect of the Earth’s axial tilt, resulting in the area within being able to have the Sun directly overhead at least once during the terrestrial year.
Because this band around the planet has the most direct sunlight, it is home to both the most humid forested areas, and the hottest deserts. On Earth, a desert is defined as an area that receives less than 10 inches of precipitation each year, which, while generally taken to mean arid hot areas, also means that most of the southern polar ice cap qualifies, as it is too cold there for precipitation. These different kinds of deserts have the highest and lowest temperatures on the planet. Within the past hundred years, Death Valley, an area of the Mojave Desert, and the lowest (below sea level) dry land point on the planet, has been known to reach almost 57C, while Antarctica has seen temperatures as low as -89C.
At sea level, on the surface of the Earth, the atmospheric pressure is 14.7 pounds per square inch (psi), called one bar by human scientists (actually, though, it’s really 1.013 bar).
This pressure varies both with height above, and depth underwater. Obviously the increase in pressure underwater is more noticeable because water is denser than air, but the differences are considerable at both extremes. Underwater, the pressure increases by one bar for every 33 feet below the surface, due to the sheer mass of the surrounding water. If you have been conducting deep-sea operations, even within pressurized vehicles, you will have to return to the surface in stages. The same applies if you are a native oceanic species that has arrived in the sea and intend to invade the land. This is because, as the pressure decreases, gas bubbles will form in blood and cell structures, which can lead to a most painful death – occasionally by explosion.
Thankfully the differences between sea level and higher altitudes are not quite so extreme.
Above sea level, the atmospheric pressure decreases on average by about 10% for every three thousand feet, though this also varies according to local weather systems that disturb the atmosphere. The lower the pressure drops, the lower the temperature needed for liquids to boil, so if your species has a high liquid content, or dependence, then beware.
In fact, the drop in atmospheric pressure at high altitudes is so acute that even human travellers on foot must stop at different levels to let their biology acclimatize to the lower pressure and different oxygen levels. This, obviously, will be the same for those visitors from parallel Earths or the past or future. Visitors from other planets who require environment suits against the Earth’s atmosphere in general should have no problem, as they will be protected anyway. Those who would otherwise be able to withstand the regular surface conditions on Earth should keep pressure suits and portable atmospheric supplies available for use in those areas where the pressure and atmospheric content levels vary.
How much actual warfare you engage in during your operations on Earth will depend both on your motivations, and the reaction you receive from the natives. However much fighting you end up doing, the golden rule to remember is that the purpose of warfare is victory, not endurance or scale.
When warfare was limited only to units of ground forces on land, and ships at sea, pitched battles of massed units were perfectly normal, as these armies were what decided the rulership of Earth’s nations. With the development of aircraft, missiles and drones, this has become less the case, at least for those nations who have such forces. Since you will be coming with superior technology, you should not actually need – unless you’re doing it wrong – to engage the enemy in massed naval or infantry battles. Swarms of troops having it out on the open field, while still an attractive idea to certain cloned armies who have no need to concern themselves about casualties, are basically a waste of time, and a sign of bad strategic planning on the part of any invader from orbit.
There will always be the need for smaller-scale engagements, such as commando raids to capture or destroy enemy facilities, but when any army in the field can be eliminated from the air or from space, by bombs, missiles, meteors, energy weapons, or whatever, then there’s really no need to land an army of your own to meet the enemy’s army. That said, if you have a particular cultural or religious requirement to meet your enemies face to face, then that is your privilege and you can feel free to do so. Naval actions should be similarly unnecessary, of course.
It will be a different matter for those of you arriving from a parallel Earth. In this instance you should simply follow your world’s or nation’s standard military doctrine for invading a neighbouring society.
Invaders from more advanced time zones, or who have come through teleportation, wormholes, or other non-starship means, will fall somewhere in the middle. It may well be necessary for you to engage terrestrial armies in the field, but your superior technology should enable victory. The rules to remember here are that, aside from having done your research, you should always be deceptive, and warfare has traditionally depended on deception and concealment for victory. Also, your logistics will be of far more importance than those of the native population, or of invaders from orbit. Invaders from orbit can always drop something on the enemy, but you who are confined to the planetary atmosphere cannot, so you must be sure that you can bring through enough materiel and reinforcements from your home before you engage the enemy.
Present-day Earth is a theatre of asymmetric warfare, rather than classical land or sea warfare. This means that the opposing forces are not equal in doctrine or strategy – one side may rely on open strategic bombing and armoured units, while the other concentrates on stealth and infiltration, for example. As a visitor to the planet, it is highly unlikely that you will just happen to be equal in mindset and ability to the human forces opposing you. You will be engaging in asymmetric warfare of one kind or another, and must know in advance which type of warfare you will conduct.
Spaceborne invaders should project their power from above to eliminate enemies at long range, while invaders able to roll massed troops and equipment through a wormhole or dimensional rift can engage in a traditional invasion, and those with little superior technology or backup from home can use stealth and infiltration techniques.
In general, however, all would-be conquerors of the Earth should find themselves in a position of conducting more quick hit-and-run raids to seize or destroy objectives, and policing actions to retain control of captured and pacified areas.
As with any other military activity, camouflage is an important part of preparing for and carrying out your invasion. It is always best in military actions to be covert, deceptive, and not seen, unless you have some honour code about being open, or an animal instinct driving your species to be obvious in order to issue challenges.
Assuming neither of those is the case, you should ensure that your forces – both vehicles and warriors – in combat are appropriately camouflaged.
This is easier for any starships you have; even without the need for energy-based cloaking devices, the sheer size of space as compared to any starship makes the ship no more than a speck at most distances, so simply preventing energy leakage from it and painting the hull black is a good way to start. On Earth, and as the individuals or materiel to be camouflaged shrinks in size, things begin to get more difficult. If you have energy-based invisibility shields of any kind, you should of course use them. If not, then there are things you can do instead. Take tips from Earth militaries, by using camouflage paint, and stealth technology.
Vehicles should be painted to blend in with the background environments in which they will be used. If possible, antimagnetic paint should be used, and any heat exhausts or power systems disguised with heat-absorbent baffles to prevent tracking by heat-seeking sensors. Radar-diffusing paints should also be handy.
As for your troops, they should also be given a means of camouflage. Personal invisibility cloaks are best, but have problematic side effects; if your eyes work the same way as human eyes, they need light to impact the retinas in order to be translated into visual information by the brain. Anything that prevents light from reaching you – such as invisibility – will also prevent this, and so render you blind. Therefore, if you have such cloaks, you will need to equip your forces using them with some other form of sensor to replace their vision.
Otherwise, either their clothing or their outer surfaces (if they do not wear clothing) should be of a drab colour to suit the background of the environment in the combat area, as per human protocols.
Once you have landed your forces on Earth, whether by means of descent from orbit, or arrival through time or across the barriers between dimensions, you will now be faced with the practicalities of mounting your campaign on the Earth’s surface.
As you know by now, 70% of the surface is covered with water, but even the remaining 30% is made up of many different types of terrain, and varies wildly in climate and navigability. Once you have boots – or treads, tentacles, or claws – on the ground, you will have to take these different environments into account.
Obviously there are different military doctrines governing actions on land, at sea, and in the air. Your choice(s) of doctrine will depend both upon where you came to Earth from, how you arrived at the planet and your intentions for it, as well as your physical nature and the environmental factors at work.
As species develop civilizations on their home planets, it is natural that their military tendencies begin with the idea of some kind of army on the planetary surface. Whether this be a Wild Hunt of semi-sentient creatures following a naturally hardwired herd instinct, or a professional army of well-trained warriors, the society’s overall fighting strategy will have evolved well before the technology for aerospace travel is developed.
As such, every military society will have evolved with the basic setting of amassing troops of peak fitness age, and deploying them to overwhelm and suppress their rival or prey societies and species.
Note that this doesn’t literally mean boots on the ground – an avian species will flock where the atmospheric conditions are suited to them, and societies that evolved in liquid environments would perhaps travel in shoals. That said, they would still be fighting bodies confined to the surface conditions of their world, and so those conditions would be ingrained.
Things change when the ability to travel out of the home environment is developed. When a land-based species develops air power, that will change its need for marching on the ground. Likewise, the shoal of marine life will have to adapt to not being able to move in three dimensions when it rolls onto dry land, and the flock of avians will also find changes when it submerges to explore the oceans, for example. If you are invading the Earth from a parallel world, or from a different time zone, but are otherwise human and native to an Earth, you should already be familiar with the principles addressed in this section. Having developed in a situation comparable to that which is known today, you will, of course, have experienced a history of evolving infantry and naval warfare at the very least.
Those of you who have crossed the stars, on the other hand, may have evolved in a completely different biosphere, or one with fewer geological, climatological, or environmental variances. In that case – and especially if this is your first invasion of a planet other than your own – you would be more advised to pay attention to the following.
Because the Earth’s surface environments vary so much, you will need specialist equipment to most effectively operate in them, regardless of your species’ nature and abilities on land.
Although aerial vehicles allow for travel to any point on the surface of the Earth, they are likely to be very resource-intensive in terms of requiring fuel. Even if the ability to fly is natural to your species, it would still, presumably, be tiring, and there’s no point in being able to swoop in to an area if you’re then too tired to fight effectively. Most importantly, however, life forms on the surface itself will be able to take shelter and conceal themselves by the simple expedient of ducking under cover.
Buildings constructed in urban areas, ground under forest cover, caves, walkways under overhanging ledges… All of these will provide cover and concealment for native life forms and resources. Sooner or later, therefore, you will have to negotiate ground terrain on Earth, either in person or in vehicles – and most likely both.
Land vehicles of some kind will, therefore, be advisable, even if your physical abilities make them not technically necessary. Whatever your abilities to adapt to the environment, or to move over terrain, it will almost always be useful to have vehicles that can offer protection from attack by human resistance forces or non-sentient animal life, move more quickly than natural mobility allows, and carry heavier weapons and supplies.
The size of the vehicle is entirely a matter of your preferred doctrine. For example, you may find that you work best operating with individual vehicles, each acting as a protective covering for a single invader, or you may prefer to deploy in transport or assault vehicles that can carry many troops and supplies to a region where you are operating with your natural mobility. This preference may or may not be influenced by the physical size of your species.
Individual vehicles may make more sense if the Earth’s gravity or atmospheric pressure is greater than that of your native sphere, or if you lack limbs entirely. Assuming you have limbs for mobility, you may find that powered prostheses will both support your frame against the environment and offer greater speed, strength, and agility than the native humans. This will provide a valuable bonus in any pursuits or melee combat encounters against humans.
If the Earth’s biosphere is totally inimical, you are going to need life support systems installed in your vehicles – of any size – and environmental controls to maintain the optimum conditions to support each of your individuals while out and about on Earth. This makes larger vehicles more logical, as more of your forces can crew each vehicle, making a better use of whatever resources are required for you to maintain the environmental systems on board.
There will always be the danger, however, of such a vehicle being damaged or destroyed, or crashing, or malfunctioning in some other way that will lead to the crew and passengers within being exposed to Earth’s biosphere. You will therefore still need emergency life-support equipment to support such stranded individuals.
This may be enough of a problem to justify using individual vehicles, which should be as form-fitting as possible on the inside, so that the exterior, whatever its form, is effectively an extension of the occupant. This should be more efficient, especially if the vehicle’s external sensors – you will need at least pressure sensors, accelerometers, and temperature sensors – are directly linked to the occupant’s nervous system. In this way, the occupant can feel their surroundings as if they were not inside a vehicle, and so will be able to navigate and respond to situations more quickly and efficiently.
The actual means of locomotion will be an important choice for vehicles of any size. Given the wide variety of differing terrains on the Earth’s land surfaces, some form of antigravity would be the best option, allowing your forces to move across any type of surface with impunity. Failing this, a form of ground effect cushion – a hovercraft, as vehicles with this feature are called on Earth – is another good choice, allowing free movement over most surfaces, and easy transitions from the land surfaces to water.
The disadvantage to this form of support is that the vehicle must be quite low, and large obstacles will block movement.
Wheels are an obvious choice, and should be as sturdy as possible. The larger the tyres, and the more variable their pressure, the more variety of surface conditions can be traversed. Lowering pressure within larger tyres will allow them to travel over far rougher surfaces, even including fallen trees or plains of stones up to the size of small boulders. Even then, slopes with large boulders or fissures, and forested areas with heavy tree growth, will still be problematic.
Segmented tracks are quite useful for covering many types of terrain, but you must be careful to ensure that the wheels are each given a separate suspension, with as much leeway as possible, especially on the vertical axis. A minimum of three wheels on each side of the vehicle is recommended, even for small single-occupancy vehicles, with the wheels arranged in a triangle. The uppermost wheel should be at least as high as the fore-and-aft wheels are apart, and if the whole arrangement of wheels on each side can also turn and/or rotate as a unit, then so much the better. Such an arrangement should give better steering and navigation ability.
If the whole triumvirate of wheels can rotate as a single unit, and especially if each wheel is able to be moved closer to or further from the centre on some telescopic mount, you should be able to arrange for a greater climbing ability, both on stairs and exterior natural surfaces.
The most versatile form of locomotion for travelling on Earth’s land surfaces, however, is a set of legs. Legs offer far, far more variability in the types of terrain they can cover, and also offer the advantage that it is possible to construct mechanical legs without having to use wheels anywhere. The joints can use spherical sockets, flexible materials, or hydraulics to move.
Ideally there should be at least three legs, and preferably four or more. Despite the bipedal nature of humanity, the bipedal form is actually inherently unstable (bipeds are designed to essentially keep moving forward, rather than even stand still). Although evolution has led bipedal species to use all of their senses in maintaining the optimum posture, it has always proved far more difficult to balance a mechanical device on two legs and make it walk with any degree of speed or stability. If you have the technology to do so, however, then feel free, as the psychological effect of seeing giant-sized humanoid figures wandering around will be effective in giving humans pause. In fact, some societies on Earth are so taken with the idea of piloted mechanical giants that they may even be persuaded to join your side just for the chance to interact with them. Failing that, they may at least be lulled into a false sense of security, and thus lured to an easy defeat while they stop to admire your handiwork.
For a better solution, four legs is more likely to be a workable system, though it would be something of a mistake to have them patterned after humanoid legs, with all four central joints aligned in the same direction. In fact, the optimum arrangement for a four-legged mechanism would have the central joints – the knees – in the opposite positions for the front and rear legs. As with the majority of quadripedal mammals you will find on the Earth, the central joint of the rear legs should be aligned to have their apex at the rear.
Even human cyberneticists have realized this, and begun to construct autonomous transport machines to this pattern, given the name ‘Big Dog’.
More legs still are also a viable option; most terrestrial insects have six, and arachnids have eight, and all are particularly manoeuvrable and adept at traversing even the most complex surfaces. Psychological effects on human witnesses and opponents can again be exploited here, as most humans have a revulsion to insectoid or arachnid forms, and utilizing such forms will not only be efficient, but frighten many humans into either not approaching to harm you, or making mistakes in their counterattacks.
You can choose whether to have weapons built in to the vehicles, or simply have the vehicle equipped with manipulator arms capable of utilizing weapons it picks up. The former approach is more reliable, with a solid mounting for whatever weapon, and means you can have the energy supply and/or ammunition supply integral to the vehicle itself. On the other hand, something capable of picking up whatever weapon or tool is necessary at the time – without the need for physical modifications – obviously offers more variety and adaptability. You will need to plan in advance whether you are more concerned with security and reliability, or with the ability to adapt to fluid circumstances.
The type of power source you use – and this applies equally to air and sea vehicles as well as those on land – should be both long-lasting, renewable, and not dependent on being refuelled or recharged in your secure areas or at home. While you will obviously not want the humans to be able to reactivate any vehicles or power units you use, there will be more chance of losing such vehicles to them if they run out of power in human-controlled areas, and can’t get back to you.
It is recommended to use one of the following means of power for your terrestrial vehicles:
1) Some form of miniature fusion reactor, or equivalent. This will be relatively clean, last for years without refuelling or recharging, and can be used as a self-destruct weapon if in danger of being captured.
2) Local fuel sources such as electric motors, diesel or petrol. These are more crude, and you may have destroyed the production facilities in any pre-invasion bombardment, but they have the advantage of not advancing the human technology if they are captured. On the other hand, they can be used against humans if captured, and, on the gripping hand, if you’re coming from a parallel Earth this will be familiar technology that will work with your existing vehicles.
3) Bioelectric or psychokinetic power drawn from the occupant(s). This has the advantage of being unstealable, lasting as long as there is an occupant, and not being subject to EMP attack or other energy-draining effects.
The most fundamental and important rule of vehicles for travelling on Earth – especially vehicles designed for a single snug occupant – however, is this: Do not rely on small wheels or castors, because there is basically nowhere on the planet, other than some of the better-maintained roads, upon which such a wheel will roll more than a few inches.
There is a big difference between space superiority and air superiority within the Earth’s atmosphere. Space superiority can be maintained with stationary vessels, and the ability to conduct orbital bombardments. Air superiority requires constant movement, either very quickly, with great agility, or in such a way to avoid detection. Air superiority means being able to control traffic in the atmosphere and use that control to exert force upon the surface and upon other atmospheric traffic.
Air superiority is a misleading subject for the tactical planners of an invasion of Earth, or any other planet. In particular, if you have a fleet of starships in orbit it may be tempting to assume that you have air superiority handled. After all, you came who knows how far, you can observe everything that happens in the atmosphere below, and you can target anything moving down there, can’t you?
Well, not necessarily. For one thing, it all depends on what weapons technology you brought with you. If you only brought strategic missiles, or mass-drivers, or some form of artillery designed for engagements between capital ships, then you will have a problem. Depending on the ship(s) you have as strategic/tactical support, your weapons systems simply may not be able to hit a manoeuvrable aircraft below, for example. Strategic weaponry simply isn’t suited to quick reactions and the flow of an aerial combat.
If you have no backup in orbit, you will also require air superiority for the strategic bombing component of your attack, if you have one. So, you are going to need to dip into the atmosphere in order to establish superiority there.
Strategic bombardment does not just mean dropping rocks from orbit, however effective that may be. Clearly you can assault surface targets with many forms of ordnance, be they bombs, missiles or meteors, and whether they be delivered from orbit, launched from surface installations, or delivered by aircraft. There are many practical reasons for the application of strategic bombardment – to destroy military or manufacturing infrastructure, prevent gathering of insurgent forces, deny areas to enemy forces, and so forth – but one thing it is not suitable for is reducing the population’s will to resist.
Historically, the military forces of various native terrestrial nation-states have often used strategic bombardment for exactly this purpose, but, historically, it has never worked to that purpose. In every recorded instance, the survivors of the bombed population have in fact simply become more determined to resist.
Therefore, use your strategic weapons as you desire for the practical purposes mentioned above – or indeed simply to eliminate large numbers of the local populace – but if your intent is to reduce their will to resist, then you will be wasting your time using this method.
For both the interception of other aircraft and fast precision attacks on small ground targets, you will need a faster, more manoeuvrable type of craft. Machines suitable for one individual occupant are best, as they can be smaller, faster, and more agile, and carry a decent weapons load.
The weapons load will vary according to whether a craft is optimized for strategic bombing, tactical ground attack, or interception of other aircraft. Currently on Earth, most airborne combats take place at long range, with missiles launched from miles – sometimes even tens of miles – away. The requirement for manoeuvrability in human-built aircraft, therefore, has more to do with the need to avoid inbound missiles than to follow a target around for a close-up attack. Depending on the nature of the weapons you mount on your atmospheric craft, this may not be the case for you.
If you fit your interceptors with light-based energy weapons, the beam should reach the target pretty much instantaneously, considering the distances involved, and so agility will be less of a requirement. If you fit projectile or particle weapons that require visible time to reach the target, or are affected by wind, gravity, or the scattering properties of moisture in the atmosphere, then you will need both speed and agility in order to get closer to your target for a higher probability of a kill.
You will also need greater agility if you intend to either rely on using part of your aeroform as a weapon – for example using a strengthened wing leading-edge as a blade to sever parts of other aircraft – or if you intend to operate at low altitudes where there is a need to avoid buildings, foliage, or geological structures.
Note that human-built aircraft tend to be relatively lightweight and therefore fragile. It does not take a large warhead or a large amount of energy impact to damage one beyond its ability to remain flying. Since your aim should be to eliminate as many enemy craft as possible to attain victory in the skies, there is no need for overkill. Calculate your weapon load to give you the best chance to hit more enemies, rather than fewer chances to do more damage to one. Also bear in mind that, with the object of victory in mind, it is the enemy aircraft you must prioritize, not the pilot (if there is one), so it is not a problem if the pilot survives the destruction of his or her machine.
You may find that you can use the same vehicles as close support craft both in and out of the planetary atmosphere, but this may present difficulties, depending on your systems of motive power and steering.
In the atmosphere, your best mode of steering a fast atmospheric vehicle will be through the use of ailerons and control surfaces built into a lifting body. In other words, you will need a vehicle design capable of being held aloft by the pressure of the air passing under it, and which can be steered by altering the surfaces in such a way as to change how much lift, or in what direction, the airflow gives you.
This is very different from in vacuum, where there is no pressure to steer against, and space-superiority fighters need manoeuvring thrusters and reaction control units for changes of orientation and direction. Your atmospheric craft will require aerofoils and rudders, and will not be able to simply spin around on the spot.
Let’s get the disappointing bit out of the way first; saucers are not a good shape for this type of craft, if you are building aircraft locally. Unless you have a solid, stable, antigravity propulsion for saucers, then leave them at home, as they will basically be suicide machines in the atmosphere, prone to flipping, tumbling, and crashing. However, if you have stable antigravity and do bring along saucers for atmospheric combat, you will find they give a psychological advantage, at least before your attack becomes widely known, as humans will find it difficult to believe your saucers are actually flying in their airspace, and be reluctant to report an engagement for fear of being considered to be hallucinating.
For high-speed aircraft, you will be better off with a delta shape, with a slim point at the front. The wingspan should be less than the overall nose-to-tail length of the craft, to reduce air resistance. For manoeuvrability of ground attack aircraft, and those operating at lower altitudes as close-support in combat areas, try larger wing areas, with a wingspan wider than the length of the craft. This will give greater stability at slow speeds, and a tighter turning circle.
You will also need a tail with a rudder for steering to one side or the other, and movable flaps on the trailing edges of the wings, for climbing, diving, and tilting to the side.
You can also use rotary-wing aircraft, or what the humans call helicopters. These use a horizontal set of tilted blades spinning at high speed to achieve lift, though most such craft will require a vertically-aligned set of blades at the end of a boom in order to maintain stability and not spin. Most such terrestrial craft have their main rotor set mounted about the fuselage, though there are variations.
If you really love your saucers, it is possible to use two contra-rotating sets of rotors at the centre of a saucer-shaped hull, though this will not have great speed or agility. All the same, the most important factor in your aerial adventures in Earth’s atmosphere is the ability to shoot down human aircraft without crashing.
Earth’s news media is replete with tales of travellers from space who are capable of traversing the infinite gulfs with their magical and near-godlike technology, but who are apparently incapable of noticing where they are in relation to the actual ground. Do not make this mistake. Do not allow the Earth natives to capture and reverse-engineer your vehicles, and to turn their crash sites into tourist attractions. If nothing else, this is embarrassing when you are trying to build a reputation as conquerors feared and respected across the galaxy.
It’s even more embarrassing if your pilots are captured, interrogated, or dissected. Note, though, that you do not actually need to have your aerial vehicles carry a pilot. Even on Earth, there is an increasing use of remotely-piloted vehicles and drones, which can be used for reconnaissance and close combat support without endangering an occupant. Since you, as new arrivals, will likely have less access to replacements for lost forces, this is a wise approach to take.
A flying machine can be replaced quite easily, as quickly as the components can be manufactured and assembled, but a pilot’s experience cannot be so easily replaced.
In total, 70% of the Earth’s surface is water, and so, while you are living, working, hunting, and fighting on the surface of the planet, you will have to be able to deal with going into areas covered in water. Obviously this will not be a problem for aquatic species, who will only need to concern themselves with the issues of salinity – whether a specific body of water is salt water or freshwater – and the Earth’s own aquatic life forms being either food or predators.
If you are a land-adapted species like humanity, or an avian species, you will obviously require, at the very least, adaptation for immersion in water. It’s reasonable to assume that if you’re capable of having built starships to come to Earth from your homeworld, then you’re also capable of building both aerial and submersible vehicles. If you can make spacesuits to provide you with a breathable atmosphere or protect against low pressures, you can make suits to protect yourselves in water.
Do not, however, be tempted to think that because you have already demonstrated the ability to build a pressurized vehicle for travelling through the near-vacuum of space, that you can simply use the same constructions to travel underwater, no matter how cool SHADO’s Skydiver combo from UFO or the saucer-shaped flying sub from Land of the Giants look when you view those transmissions. Trying to pull off the combination is probably unwise, though not necessarily impossible.
The structure of a mechanical construct built to withstand the absence of pressure is of necessity very different to the structure of something designed to withstand increasing external pressures and the load bearing of immense tonnage of water. At base, a vehicle designed as a submersible will have to be built with a greater attention given to internal supports and bulkheads more evenly spread throughout the design, as opposed to being designed to withstand g-forces at launch, and then zero pressure outside thereafter.
The deepest underwater point on the Earth’s surface is 36,200 feet down. That means the pressure there will be 1,097 bar, 1,097 times the pressure of the atmosphere at surface level. The average depth of the Earth’s oceans is around 14,000 feet, or 2.65 miles, with a pressure of 424.24 times the atmospheric pressure at sea level. This means any submersible vehicle must be designed and built to protect its interior from that level of external pressures.
Since the mechanical engineering requirements are very different, it would make far more sense to design and use separate vehicles for these environments. It would not be impossible to build one that could be used in the full range of pressures, but the cost-effectiveness in resources would be far less.
You will not need antigravity or the like to power submersible vehicles, or to allow them to ascend and descend. Flooding buoyancy tanks with water will allow descent, and flushing them with air will enable ascent. A submersible or surface boat can easily be powered by any form of storage batteries (so long as they are of the right scale) or power generator, nuclear or otherwise. On the surface, sails are still effective, and used across the globe. This is not a fast or highly technological means of propulsion, but excellent for silent movement and energy efficiency. Under power, a simple screw propeller, or multiples thereof, is still traditional both on surface and submersible vehicles. Propellers cause turbulence in the water, however, called cavitation (it leaves brief cavities, or holes, in the water), and because sound waves propagate even more effectively in the density of water than in air, this makes vessels detectable. A good solution, therefore, is to use some kind of hydrodynamic drive, drawing water in at the front of the engine, putting it under pressure, and squirting it out the rear, as per a jet or rocket engine. As well as being stealthier, this also makes for a smoother ride, and the relative lack of moving parts means there’s less chance of catastrophic mechanical failure.
It is also worth considering using the Earth’s magnetic field as a power source. It varies geographically and according to the local geology, but is always strong enough that you could use a magnetic drive to move along or across the field. This would be useful if you are using vehicles designed for multiple environments, as it would work equally well on land, sea and in the air. It could be considered a sort of poor man’s antigravity, so long as the vehicle remains within the effect of the Earth’s magnetic field.
If you’re visiting by means of wormhole or dimensional portal, you’ll want to make sure that the end destination isn’t under water, unless you’re from an aquatic species, of course. That said, transitioning to an underwater arrival point would be an effectively camouflaged way of arriving, so long as you are able to ensure that the displacement of the local water does not cause visible surface disturbances, or enough turbulence to affect the passive sonar of ships, or to trigger the various underwater early warning sensors that are positioned on the ocean floor in various areas.
To pull this off, you will have to plan very carefully, well in advance, so that you have worked out how to arrive smoothly. Then again, this is true of any form of arrival at a target you plan to attack, so it should be second nature by this point.
In Earth’s history, naval doctrine became vital as human civilization rose, because it was necessary to control the bodies of water in order to carry armies and trade goods. Since the land surface of the planet is so uneven, waterways were much easier options for long-distance journeys. Even now that air traffic is rife, and roads on land have been paved and metalled, and so are able to safely carry massive amounts of traffic and freight, the seas are still a vital part of global travel and trade.
The reason for this is simple; ships can be pretty much any size, and so can transport far more people or cargo than anything else on the planet. This means that you must pay attention to sea traffic and shipping, because it will be used to transport military equipment from areas where you (as yet) have no presence, to areas where the human resistance needs weapons and materiel to use against you.
You’d think that such large ships would be easy to detect from orbit – or even from the air – and destroy, but you might also be surprised. There are 225,625,000 square miles of ocean to search. That will take a lot of processing power to analyse the results your sensors trained on the oceans give you. If you have such good sensors, and enough processing power to spare to analyse their output, good for you – go for it.
Otherwise, it helps to narrow the field somewhat. Thankfully, even on a planet with such a large expanse of open ocean, there are busy areas and bottlenecks, where larger numbers of ships tend to congregate or travel through, and these areas will make excellent killing zones for attacking human shipping.
Some of these areas are busier because they are close to major ports and land-based transport hubs from which arriving cargo can be distributed, or departing cargo can be loaded. Others are busy because they are safe channels that avoid dangers such as underwater reefs or expanses prone to storms.
The best place to attack shipping, therefore, is in the vicinity of ports. The busiest ports in the world are those of Shanghai in China, Kobe in Japan, Hong Kong in China, Dover in England, Rotterdam in Holland, and Los Angeles in the USA. A meteor or nuclear strike on each will largely disable humanity’s ability to ship materiel by ocean. Doing the same to the Suez and Panama Canals would also be advisable.
The busiest areas of open ocean are the English Channel, the Caribbean, the Arabian Gulf, and the South China Sea, especially around Indonesia. In fact the busiest part of the Caribbean is sometimes called the Bermuda Triangle, and is famed among humans for the number of mysterious disappearances of ships in the area. Most humans seem to be of the opinion that your forces are already at work there, but, in reality, the percentage of shipping passing through the area which vanishes is actually no greater than the percentage of shipping that goes missing globally. It is simply that the area is so busy that there are more ships, so more vanishings.
The existence of these busy shipping lanes is what will make the use of sea power advisable as part of your campaign on Earth. Simply dropping a meteor or nuke on the area will give no guarantee that you will destroy enough ships to prevent them resupplying human resistance. Therefore you will need to blockade these areas, and engage and destroy seagoing vessels as they enter and pass through.
Now, it is true that powered flight will get you and your forces to any point on the globe quickly, and that vessels stationed in orbit can be placed above any point you desire to observe, but a descent from orbit to the surface will still take discernable time, and powered flight, or a starship placed on station-keeping low in the atmosphere, will require a constant fuel supply. The correct shape of hull, however, constructed from the appropriate materials, will simply float wherever you put it on the liquid surface of the planet. You will require ships or submarines.
You can bring your own, build them locally, or even simply commandeer existing ones from their present owners.
Submarines will be the most valuable to you, since these will be able to track and destroy shipping in the blockaded areas quickly, and with less likelihood of detection. Any form of large breach to a ship’s hull below the waterline will deal with it quite effectively. If you are used to operating in starships you should be used to the idea of working and fighting entirely within a closed environment that protects you from an inimical exterior, so attack submarines should be quite psychologically comfortable for you.
Capital warships such as battleships will be of little or no use to spaceborne invaders, though their ability to launch missiles and bombard targets many miles inland may be desirable for those who are on a technological level similar to that of Earth in the current time zone. Your own starships should be able to deal with such targets more effectively, and slow-moving surface ships will be easy targets for missiles and air attack. Smaller and faster warships are more useful if you need to board shipping or otherwise conduct operations on the water’s surface, though it is recommended that they be protected by orbital or aerial vehicles as cover if possible.
Aircraft carriers may or may not be of interest to you. They are large and relatively slow targets that need protection, but they would also be capable of hosting your atmospheric craft if you wish to have them mobile closer to the surface, rather than based in an orbiting mothership.
Transport shipping is definitely worth considering. Spaceborne invaders do not really need a naval doctrine on the surface, as your projection of power will be from orbit to planetary surface, but other, purely surface-based, invaders will require to be able to move fleets around to transport forces and cargo from conquered areas to unconquered ones. Also, everyone, even spaceborne arrivals, will find that surface ships can store and carry more for less energy expenditure than anything else, which is always useful, and enables materiel such as weapons to be stored on the surface ready for quicker deployment. Such ships would also, once air superiority is established, and humanity’s submarines and missiles are eliminated, make excellent staging areas on the surface, out of reach of resistance forces.
Considering that huge percentage of the planet’s surface that is water, ruling the Earth still means ruling the waves.