The devastation of Japan’s shipping had an enormous impact. Soldiers were deprived of food and medicine. Aircraft did not get spare parts and could not launch missions. Troops did not reach the places they were sent as reinforcements. On March 12, 1944, a broken 2468 message gave the route and schedule of the Twenty-First Wewak Transport convoy, sunk while leaving Wewak to return to Palau. When the Japanese Eighteenth Area Army made a “complete tabulation of shipping from Rabaul and Truk during January,” in an attempt to convince Japanese Army headquarters that it was feasible to send them much-needed supplies, these messages laid out the shipping routes and sealed their doom. Only 50 percent of ships reached the destination; only 30 percent got home.
At the end of the war, a U.S. naval report found that “more than two-thirds of the entire Japanese merchant marine and numerous warships, including some of every category, were sunk. These sinkings resulted, by mid-1944, in isolation of Japan from her overseas sources of raw materials and petroleum, with far reaching effects on the capability of her war industry to produce and her armed forces to operate. Her outlying bases were weakened by lack of reinforcements and supplies and fell victim to our air, surface and amphibious assaults; heavy bombers moved into the captured bases.” This report’s author, C. A. Lockwood, commander of the submarine force of the U.S. Pacific Fleet, noted that his men got a “continuous flow of information on Japanese naval and merchant shipping, convoy routing and composition, damage sustained from submarine attacks, anti-submarine measures employed or to be employed, effectiveness of our torpedoes, and a wealth of other pertinent intelligence.” Whenever code breaking was unavailable, he added, “its absence was keenly felt. The curve of enemy contacts and of consequent sinkings almost exactly paralleled the curve of volume of Communication Intelligence available.”
He added: “There were many periods when every single U.S. sub in the Pacific was busy” responding.
In fact, he added, code-breaking intelligence made it seem to the Japanese that there were more American submarines in the Pacific than there really were. “In early 1945 it was learned from a Japanese prisoner of war that it was [a] common saying in Singapore that you could walk from that port to Japan on American periscopes. This feeling among the Japanese was undoubtedly created, not by the great number of submarines on patrol, but rather by the fact, thanks to communications intelligence, that submarines were always at the same place as Japanese ships.”
The commander noted that dispatches that led to attacks usually had to be destroyed, so he took it upon himself to list just a few of the more notable achievements. They included the sinking of aircraft transport Mogamigawa by the submarine Pogy in August 1943; the sinking of escort carrier Chuyo by Sailfish in December 1943; and so on and so on, including “the contact and trailing of Yamato task force by Threadfin and Hackleback in April 1945, which resulted in sinkings the following day by carrier air forces of the battleship Yamato, the cruiser Yahagi, and destroyers Hamakaze, Isokaze, Asashimo and Kasumi.” He pointed out that code breaking was responsible for at least 50 percent of all marus sunk by subs. And “information concerning enemy minefields” enabled U.S. subs to avoid them, and forced Japanese ships “into relatively narrow sea lanes.”
Arlington Hall worked closely with “Central Bureau Brisbane,” its satellite unit in Australia, and with Australian and New Zealander code-breaking allies. CBB stayed in touch with the staff of General MacArthur, and a memo in 1944 noted how the breaking of all the Japanese Army codes—shipping, administrative, air force—contributed to the success of Operation Cartwheel, MacArthur’s island-hopping campaign. Thanks to the penetration of most Japanese Army systems, it said, MacArthur knew about supplies, troop training, promotions, convoy sailings, reserves, reinforcements, and impending attacks. By May 1944, messages translated by Arlington Hall had alerted the U.S. Army to changes in the makeup of the Japanese Army, helping identify new armies, divisions, and brigades. The staff knew how many planes the Japanese Army air forces had and the condition of the railroads; they knew about shipping losses and were able to keep a running tally. Citing a few of the biggest achievements, the memo noted that “never has a commander gone into battle knowing so much about the enemy as did the Allied commander at Aitape” on July 10 and 11, 1944.
This is the kind of code-breaking intelligence Pacific commanders received: medical reports, incidence of disease, number wounded, percentage of total strength, casualties, losses incurred through submarine and plane attack, convoys delayed, marus sunk, brigades shipwrecked, and tools, arms, machinery, and codebooks lost. As the Americans planned to retake the Philippines, code breakers fed them information on reinforcements, plans to hamper U.S. air activity, units engaged in battle, army supplies, and reinforcement problems.
The code breakers also responded to requests from American military intelligence. “When they were planning some major moves against the Japanese—either against some of the islands or the last big move that they were planning was, of course, the invasion of the Japanese mainland—They would come and ask us, if possible, to concentrate on messages from one or two certain places,” remembered Solomon Kullback. As a result, MacArthur “wasn’t going in blind into a lot of these areas he invaded.”
Code breaking also proved instrumental in reducing American casualties. George C. Kenney, MacArthur’s air corps commander, was able to prevail in the air and shorten the ground war. Code breaking enabled the destruction of Japanese aircraft in Wewak in August 1943, and in Hollandia in March and April 1944, making possible MacArthur’s “greatest leapfrog operation,” along the northern New Guinea coast. In November 1944, Arlington Hall decoded messages saying that two convoys contained troops to reinforce the Philippines. The U.S. Navy sank at least six of the ships and disabled one, and one caught fire.
Joe Richard, the young officer working in the Australian unit who spotted the telltale digit pattern that led to the break in 2468, told later of how the recovery of a codebook on Okinawa, in June 1945, gave them all the translations for that period and “led to reading about the Japanese army’s preparations to fight against any landing on their home islands. These were so extensive, involving every Japanese, that the Allied general staff estimated (based on experience at Iwo Jima and Okinawa) that 1 million casualties might be expected by our allied forces, which I think induced Truman to use the atom bomb and to moderate FDR’s unconditional surrender ultimatum and accept Japanese surrender keeping the Emperor.”
In the summer of 1944, the U.S. military retook Guam. The Americans got an intercept station up and running again, and Dot Braden, sitting at her wooden worktable, began to get a lot of intercepts from Guam. Never having been anywhere near the Pacific Ocean, she always visualized rather fancifully that Guam was a tiny little island with a single palm tree on which a lone American GI sat, sending intercepted messages over teletype, which ended up in her hands.