AUTHOR’S NOTE
General MacArthur … was better informed about the enemy dispositions than any other military leader in history.
The Pacific War
John Costello
The midget submarine operations on Sydney Harbour and Madagascar, although they achieved only minor military results, had been intended by Admiral Yamamoto to play a significant role in Japan’s overall naval strategy. However, his main strategy was to launch an operation on the Aleutians and Midway Island. He gambled on trapping the American Pacific Fleet, which would steam west from Pearl Harbor to repel the Midway operation and be caught unaware by Japanese submarines and carriers strategically lying in wait. But the gambit failed because the Allies were reading a large portion of crucial Japanese fleet signals and, significantly, over-the-target weather reports sent in low-grade code, which provided an indicator of intended area operations. Although only 20 per cent of all this traffic was deciphered, it was enough to anticipate Japan’s next move in the Pacific theatre.
The bulk of the information on the secret war was gleaned from listening posts at Washington’s Naval Headquarters (NEGAT), Pearl Harbor’s Pacific Fleet Combat Intelligence Unit (HYPO), and MacArthur’s code-breakers in Australia (BELCONNEN). Of these three intelligence units, BELCONNEN was used to the greatest advantage by MacArthur who has been described by John Costello in his book, The Pacific War, as being “better informed about the enemy dispositions than any other military leader in history”.
Prior to the war in the Pacific, “Magic” machines had been installed in Washington and Manila, with the unit destined for Pearl Harbor diverted to Bletchley Park in order to read Japanese signal traffic. When MacArthur fled from the Philippines to Australia in March 1942, he carried with him the decoding device, which he quickly set up in his Melbourne headquarters. Code breakers attached to MacArthur’s command searched continuously for patterns in the Japanese signal traffic in order to break their codes. A steady stream of intercepted information was exchanged between Washington, Pearl Harbor, and Melbourne to quickly build up a directory of solved cryptograms. Within five months of the Pearl Harbor attack, the highly sensitive and revealing Japanese JN25 code was sufficiently penetrated to allow MacArthur and Rear-Admiral Chester W. Nimitz in Hawaii to obtain a clear picture of Japanese disposition and intentions. Fortunately for the Allies, at this crucial period of 1942, a planned change in the two code books that made up the JN25 code, was postponed from1 April to 1 May, and was later deferred to 1 June because of problems associated with distributing the two new books to every Japanese ship.
The code breakers began to reap the rewards of their efforts when they deciphered Japanese intentions of a southward thrust towards Australia as early as 25 March. By April, the volume of traffic focusing on Rabaul had intensified and, coupled with coast-watcher reports of a sudden increase in the flow of planes, shipping and troops to Rabaul, MacArthur and Nimitz were able to predict that the Japanese intended to launch an offensive operation in the South Pacific in the near future. By 3 May, the entire Japanese plan for the sea invasion of Port Moresby and Tulagi had been deciphered and Nimitz deployed his meagre naval forces accordingly to intercept the Japanese threat.
“Magic” intercepts had also uncovered plans for a second offensive operation in the mid-Pacific before the main action of the Coral Sea battle had unfolded, although some confusion existed over the precise location. On 5 May, Yamamoto issued his operational plans for “Operation MI” (Operation Midway) with the aim of occupying Midway and strategic points west of the Aleutian Islands. His strategy contemplated a crushing and decisive naval engagement ending with the defeat of the American Pacific Fleet, which, he hoped, would bring the United States to the peace table and a quick end to the Pacific War.
Yamamoto’s elaborate plan was to be choreographed in a series of carefully synchronised movements centred on Midway. His strategy called for the Aleutian Islands operation to precede Midway by 72 hours. The Japanese fleet then intended to lie in wait for the United States Pacific Fleet, which would predictably come charging to the battle area from the west where it would be mauled firstly by a submarine force deployed across its line of advance, and secondly by a surface action from aircraft launched from four Japanese carriers.
The first deciphered signals referring to the Midway operation were intercepted on 4 March 1942; by the end of April signals were being intercepted referring to the Aleutians and that the operation would occur after the Port Moresby invasion. However, the Japanese signals used the letter indicators AF, representing Midway, and AO for the Aleutians, which confused the intelligence teams working desperately to solve the riddle.
By the end of April, MacArthur’s BELCONNEN intelligence team in Australia insisted that the Japanese intended to invade Australia, and not Midway or the Aleutians as was considered by Nimitz’s HYPO team in Pearl Harbour. He demanded reinforcements to meet the threat, but they were denied him because of the “beat Hitler first” global strategy. MacArthur provoked the wrath of Churchill when he persuaded Curtin to demand at least two Royal Navy carriers and first call on reinforcements on their way to the Middle East. He then leaked reports to the Sydney Morning Herald in late April 1942 about the forthcoming Operation in New Guinea, which sent the Chief of Naval Operations in Washington, Admiral Ernest J. King, into a rage for endangering the whole naval intelligence operation. Fortunately, the Japanese paid little attention to the newspaper speculation, which became evident when radio intercepts showed no discernable change in Operation MO that had now been extended to take in the phosphate-rich Ocean and Nauru islands once Port Moresby was seized. Prior knowledge of a midget submarine attack on Sydney Harbour, and a major air strike on Townsville following Operation MO, would have only strengthened MacArthur’s belief that the Japanese planned to invade Australia.
Despite MacArthur’s fears, intercepts soon indicated a Japanese naval force amassing at Saipan Island for an operation in the mid-Pacific, but Nimitz was at his wits’ end to pinpoint the exact location of the intended offensive. Washington’s NEGAT intelligence team further confused the picture when it concurred with MacArthur’s predictions of a renewed southward advance. Uncertain now of where the Japanese intended to move, Nimitz’s HYPO team decided to initiate a ploy to once and for all determine where Yamamoto’s forces intended to strike. Instructions were relayed to the garrison commander on Midway to make an emergency radio call in plain English stating the island’s water distillation unit had broken down. Naval authorities radioed back from Pearl Harbor that a water barge was on its way with emergency supplies. The ruse succeeded when, 24 hours later, intercepts were deciphered containing orders for the Japanese invasion force to take aboard extra water.
Good fortune continued to favour the BELCONNEN, HYPO and NEGAT intelligent teams until 1 June when the Japanese finally switched to their new code books; but the exact plan and time of the Midway operation had already been exposed and Nimitz carefully deployed his carrier force in ample time to successfully ambush the Japanese carriers. As a result, the tide of the Pacific War was turned sharply in favour of the Allies.
Despite the adverse outcome at Midway for the Japanese, Yamamoto continued as Commander of the Combined Fleet through the following Guadalcanal campaign, which further depleted Japan’s naval resources. While on an inspection tour in the Northern Solomon Islands, he was killed in an aerial ambush by US Army Air Force planes on 18 April 1943 following a signal intercept five days earlier. Since most of his inspection trip was to be made by minesweeper, it is indicative of how precisely timed the American plan had to be. His death was a crushing blow for the Japanese Imperial Navy.
In an Australian context, in 1942 Australia found herself almost defenceless. Japanese war plans called for the seizure of Fiji and New Caledonia and carrier strikes against Australian east coast cities in July 1942. Only the miracle of Midway in June and victory in the long and terrible struggle for Guadalcanal pushed back the Japanese threat.
The Battle of Midway in June 1942 was the turning point in the Pacific War. From then on, Allied forces slowly won back the territories occupied by Japan. In 1944, intensive air raids started over Japan, and in the northern spring of 1945, US forces invaded Okinawa in one of the bloodiest battles of the Pacific War. After US military forces dropped two atomic bombs on Hiroshima and Nagasaki, Japan finally surrendered unconditionally on 14 August 1945.