Press the pursuit, Scott ordered. Give the enemy no chance to reorganize and stand. Drive straight through to Mexico City if possible. In obedience, troops in blue surged forward.

But the Mexican army possessed an amazing ability to recover and rally even after a disastrous defeat. Holding Churubusco would grant them respite - would allow Santa Anna an opportunity to marshal his still considerable strength, 16,000 men, to protect the capital. So the general in chief and president of Mexico commanded the hard core of veterans, who were Churubusco’s garrison, to defend their post to the last man.

The Americans could have bypassed that fortified town. Yet if they did, they would leave behind them what is called in military terms a “hedgehog.” Attacks like sharp quills from the animal could be launched from it against the invaders’ rear. On August 20, 1847, Scott issued a new order: Storm Churubusco.

Churubusco, whose name was derived from an Aztec word meaning “place of the war god,” would soon live up to its name. Assaults had to be delivered over a ditch-flanked causeway leading to a stone bridge across a river canal. Every inch of the approach could be swept by cannon fire. Beyond rose the massive walls of the Convent of San Pablo with its chapel and garden. It had been turned into a fort. And Americans would shortly be made aware of the grim fact that most of that formidable array of artillery was served by crack gunners, the San Patricios under ex-Sergeant Riley.

Famous old American regiments, with gallant records in the Revolution and in the War of 1812, advanced along the causeway. Deadly cannon fire swept them back. A deluge of solid cannonballs - round shot - poured down on them. From the muzzles of the Mexican guns burst deadly sprays of grapeshot - small iron balls clustered around a rod like a bunch of grapes on its stem - and of canister. The Mexican fire blew up two American caissons (ammunition wagons) with a roar and cut down gun crews. More artillerymen in blue were picked off by sharpshooters in a church tower.

The defenders at the bridgehead and their assailants were so close to each other that they were able to distinguish each others’ faces through the battle smoke. The San Patricios, sighting along their gun barrels, recognized some of their former officers leading the assaulting troops. At the shortened ranges, the expert gunners of the Red Company could use their cannons as sniping weapons. They loaded their guns with canister, aimed” carefully, and fired.

The San Patricios were the backbone of the defense. Their deadly fire accounted for a large proportion of the considerable American losses which rose to 139 killed, 876 wounded, and forty missing. Santa Anna later declared: “Give me a few hundred more men like Riley’s and I would have won the victory.”

Again and again the Americans attacked and as often were flung back with heavy losses. Widened lines strove to close in on other sides of the stronghold, thrusting through the tall corn and wading the drainage ditches. Volleys and sorties by the garrison smashed them back.

Gradually, stubborn American valor and keen marksmanship began to tell. One force made a circuit, forded a canal, and advanced to seize the road to the capital. Plainly it could not be checked for long. As it narrowed the avenues of escape, a trickle, then a stream of Mexicans commenced retreating while the way was still open.

The defenders of the stone bridge began to waver. American troops swarmed down on the bridge and stormed it at bayonet point. Mexican guns captured there were swung around to pound the walls of the convent. Breaches gaped in its masonry. The Mexican infantrymen in its garrison started to lower the colors and hoist the white flag of surrender. Before their hands could touch the halyards, knives were plunged into their backs by the San Patricios.

Grim Riley and his battalion of American deserters would allow their Mexican comrades neither to surrender nor to retreat. The San Patricios well knew that betrayal of their country had placed hangmen’s nooses around their necks. Better die fighting than be taken prisoner and dangle from a gallows.

The breaches in the convent’s defenses widened. The fire of the defenders, their ammunition dwindling, slackened. Cheering, Scott’s assault columns crashed through into the convent garden.

Now a strange event raised the combat to the height of ferocity. Among the storming American troops was a unit called the Spy Company, composed entirely of renegade Mexicans. Robbed or wronged in other ways by their own people, they had - like the San Patricios - joined the enemy and had proved as valuable as scouts to the Americans as Riley and his men had been to the Mexican artillery. So in the last clash in the convent, traitors fought traitors, battling with hatred and fury. American infantry by the side of the Spy Company attacked the enemy just as mercilessly.

By scores the San Patricios fell. A few managed to escape. At length, out of a battalion of 260, only Riley and seventy-four of his gunners were taken alive.

Santa Anna’s army streamed away in flight, hotly pursued by American infantry and dragoons whose sabers slashed, cutting down fugitives. Captain Phil Kearny led two of the squadrons harrying the retreat. Though trumpets sounded the recall, he galloped on with two squads of his own gray horse troops whose mounts he had bought with his own money to assure their uniform color. On they rode, straight to a gate of Mexico City. As they burst through, a cannon blast mangled Kearny’s left arm. His cavalrymen supported him, reeling in the saddle, while they wheeled and rode back. Kearny recovered, but his arm had to be amputated. A general in the Civil War, he rode with his reins in his teeth, his right arm wielding a saber. He met the death that had missed him in Mexico at Chantilly, Virginia, during the Civil War.

The American army gained a great victory at Churubusco. Four thousand Mexicans were killed or wounded; those casualties plus prisoners and other missing whittled down Santa Anna’s strength by one-third. Captures included eight generals and thirty-seven cannons. But the cost of victory was far from cheap for the Americans: sixteen officers and 139 enlisted men were killed, while sixty officers and 816 rank-and-file were wounded.

But the last barrier on the road to Mexico City had been smashed.

The fate of the survivors of the San Patricio Battalion was soon determined. Eight-pound, 3-prong iron collars were riveted around their necks, and they were held under heavy guard for a court martial. In a fair trial, they were convicted and given sentences that were strictly in accordance with the military laws of the period, though Mexicans fumed that the punishment was Gringo barbarism. Riley and others, who had deserted before the commencement of hostilities, were ordered to be whipped and branded. The remaining fifty San Patricios were condemned to be hanged for desertion in time of war.

Ex-Sergeant Riley, who was lashed and branded with a “D” for deserter, would labor as a convict as long as the army remained in Mexico. Then, head shaved and buttons stripped from the uniform he had dishonored, he was drummed out of camp while fifes piped the “Rogue’s March.”

One group of the deserters who had been condemned to death was executed soon after their trial. Riley and the others who had been spared were forced to dig their graves. Then the condemned men, nooses around their necks, were placed in carts which were driven out from under long gallows. The rest of the condemned men were held until the American army made its final assault on Mexico City.

Today in a suburb of Mexico City stands a cross in memory of the San Patricio Battalion. It is marked with a gamecock, symbol of their bravery - with dice, token of their gamble with fate - and with an emblem of the stake they risked: the skull and crossbones of death.