Early in 1944, the United States military set their eyes on the Marshall Islands after landing on Kwajalein Atoll. To help with the attack, on February 17, 1944, they bombarded Truk, Caroline Islands, due to its proximity to the Marshall Islands. During this bombardment, the Japanese lost 300 aircraft and some 200,000 tons of merchant shipping.
With the impending invasion of the Marianas Islands, Admiral Mineichi Koga, on March 8, sponsored Operation Z, as a defense against the American attack. This plan was approved by the Imperial Japanese Navy General Staff, with a formal title of Combined Fleet Secret Operations Order No. 73. The operation called for the utilization of the remaining land-based aircraft numbering approximately 1055 in the Marianas, the Caroline Islands, and in western New Guinea. The plan's success also called on a naval force of around 9 aircraft carriers and their 450 or so aircraft assisting in the defense. The plan was intended to create the Kantai Kessen, one final battle that would devastate the American forces and force them to sue for peace.
Cessation of operation
Throughout the spring of 1944, the aircraft losses for the Japanese continued to mount which was severely endangering the success of the operation; however the death knell occurred on March 31, 1944, when Mineichi Koga and some of his staff were killed in two separate plane crashes, while the remainder were captured. Among those captured was Shigeru Fukudome, who was in the second plane. After extricating himself from the wreckage, he was able to locate land and began to swim ashore. During this portion of his ordeal, he kept the briefcase that contained the top secret documents concerning Operation Z. After spending hours in the ocean, all the while approaching the coastline, he was spotted by some Filipino fishermen. Not sure if they were friendly or rebels, Fukudome released the briefcase with the intention of allowing it to sink; however, as he was picked up, the local fishermen noticed the slowly sinking briefcase and brought it aboard. Upon reaching land, the documents were sent to the American forces for analysis. A clandestine high-priority submarine pickup was arranged, with a cover story of evacuating American refugees. The submarine picked up the documents, along with 40 American men, women and children. Traveling mostly on the surface for speed, and diving only when needed, the submarine survived depth charging twice, arriving near the American naval base in Darwin, Australia on May 19. From there, the documents were flown to Brisbane. The Z Plan documents were in plain text, rather than code, and were translated on an urgent basis by the top five translators at the Military Intelligence Service attached to the Allied Translator and Interpreter Section, it was first translated by Yoshikazu Yamada and George "Sankey" Yamashiro, two nisei translators. Copies of the translation were rushed to General Douglas MacArthur, who quickly forwarded them to Admiral Chester Nimitz, commander-in-chief of the Pacific Fleet. Among other things, planned Japanese diversionary tactics were now anticipated by the Americans, leading to the lopsided American victory in the Battle of the Philippine Sea, the largest aircraft carrier battle in history, and one of the decisive battles of the Pacific war. After the loss of the plans, Admiral Toyoda Soemu rewrote the plans in a more aggressive manner and named it Operation A-Go.
