Z Plan (Japan)

The Z Plan (Japanese: 新Z号作戦) was the Japanese plan of attack in the case of an American invasion into the central Pacific ocean. The plan was the final attempt by the Japanese to bring about the Kantai Kessen, the decisive battle that would be the avenue of victory for Japan. The capture of these plans were an important military intelligence achievement in the Pacific War.

The Z Plan was developed under the leadership of Admiral Mineichi Koga, commander in chief of the Japanese Combined Fleet, and was approved by the Imperial Japanese Navy General Staff. The plan was formally titled: Combined Fleet Secret Operations Order No. 73, with the final version issued on March 8, 1944. On April 1, en route to Davao, Admiral Koga's plane crashed into the ocean during a bad storm, killing Koga and everyone else aboard. Admiral Koga's chief of staff, Rear Adm. Shigeru Fukudome, carrying a copy of the Z Plan, was traveling the same itinerary in a separate plane. This second plane crashed near Cebu Island after attempting to avoid the same storm. Fukudome and nine others eventually made it to shore, where they were captured by local Filipino guerrillas who were supportive of the U.S. forces.

Because of the loss of the two planes, with their high-level occupants and important documents (Japanese ocean-current experts believed documents would eventually wash-up on shore), the Japanese applied intensive pressure on the local population. Partially as a result of this pressure, Fukudome and the others were eventually released.

On April 3, a wooden box from Fukudome's plane washed up near shore, and was secretly recovered by two villagers. The box contained the Z plan, a bound document in a red leather portfolio, with a Z on the cover. The documents eventually made it into the hands of the local guerrilla organization, the Cebu Area Command, under Lt. Col. James M. Cushing. The ornate nature of the documents led the CAC and their American handlers to suspect the documents were very important, a suspicion that was later reinforced by Japan's offer of a large reward for the return of any documents.

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 (and Admiral Kogo's counterpart). 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.