Fortunately for America, the secret battle plan was coded in plain Japanese.
May 19, 1944: Japanese American Military Intelligence Service linguists began translating the Japanese Navy’s secret “Z Plan.”
In May 1944, three Japanese aircraft were flying during an enormous storm near Cebu in the Philippines. The storm proved devastating for the Japanese.
Admiral Mineichi Koga, who was just promoted to the commander in chief, and some of his staff were killed when the first plane crashed.
When the second plane also crashed, the chief of staff Admiral Shigeru Fukudome was captured. But the plane had something else with it: a briefcase with highly classified military documents, called the “Z Plan.”
Fukudome didn’t bother looking for them during the crash. He was convinced the documents went down with the plane. He was wrong.
Local Filipino villagers recovered the documents floating ashore.
Courtesy of U.S. National Archives and Records Administration
A Japanese flying boat, similar to the one involved in the March 1944 storm near Cebu that killed Admiral Koga and led to the loss of the Japanese Navy’s secret “Z Plan.”
Courtesy of U.S. National Archives and Records Administration
Admiral Mineichi Koga, commander-in-chief of the Imperial Japanese Navy and architect of the secret “Z Plan,” shortly before his death in a storm near Cebu in March 1944.
Courtesy of U.S. National Archives and Records Administration
Rear Admiral Shigeru Fukudome survived the March 1944 plane crash carrying Japan’s secret “Z Plan.” He believed the classified documents had sunk with the wreckage.
Courtesy of U.S. National Archives and Records Administration
USS Crevalle, one of the American submarines used to secretly transport recovered portions of Japan’s “Z Plan” through the Pacific after the documents fell into Allied hands.
U.S. Army Photo
Members of the Allied Translator and Interpreter Section, the unit that translated captured documents, interrogated prisoners, and analyzed military communications.
U.S. Army Photo
Japanese American Military Intelligence Service linguists attached to ATIS studying captured Japanese documents in the Pacific during World War II.
The Z Plan
The documents were critical.
Admiral Koga’s strategy was spelled out in the Z Plan. It outlined defensive plans against Allied attacks on Japan’s South Pacific possessions and made provisions for engaging the American fleet in a decisive battle, including naval strategy, fleet movements, and aircraft deployment.
When the Japanese government found out about the plane crash, they became desperate to locate the briefcase with the documents and launched a massive search.
Meanwhile, the Z Plan binder slowly made its way to the American forces. Within days, it was in the hands of Allied Translation and Interpreter Section (ATIS).
It was an intelligence goldmine. But first, someone had to read it.
Not Even Coded
Fortunately for the United States, the documents were written in ordinary Japanese rather than encrypted code.
That mattered because America already had a secret weapon of its own: Japanese American Military Intelligence Service (MIS) linguists.
Many of the Nisei translators working for the MIS had spent years studying military terminology, naval language, dialects, and Japanese writing styles. Some had family members incarcerated in American concentration camps while they themselves served the U.S. military overseas.
The top-5 MIS translators attached to the ATIS, including Yoshikazu Yamada and George “Sankey” Yamashiro, worked quickly.
Two Nisei soldiers of the MIS interrogate a captured Japanese prisoner. Armed with language skills and cultural fluency, they helped to save lives and shorten the war.
Z Plan revealed detailed Japanese naval strategy, aircraft strength estimates, and preparations for a decisive battle against the American fleet in the Pacific.
Courtesy of Mitsuko Sankey
George “Sankey” Yamashiro, one of the top Japanese American Military Intelligence Service linguists attached to ATIS, helped translate the captured “Z Plan” documents.
The title page of the translated “Z Plan” documents prepared by the Allied Translator and Interpreter Section (ATIS) on May 23, 1944.
Courtesy of U.S. National Archives and Records Administration
Intelligence gathered from the translated “Z Plan” helped American commanders, including Admiral Chester Nimitz, prepare for the Battle of the Philippine Sea.
Official U.S. Navy Photo
Japanese aircraft shot down during the Battle of the Philippine Sea in June 1944, one of the most decisive American victories of the Pacific War.
Tremendous Advantage
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.
The intelligence gathered from the Z Plan played an important role in preparations leading up to the Battle of the Philippine Sea in June 1944, where Japan suffered devastating losses in aircraft, pilots, and carriers.
American forces would later nickname the battle: “The Great Marianas Turkey Shoot.”
It became the largest aircraft carrier battle in history, one of the decisive battles of the Pacific War, and one of America’s most lopsided victories.
But the Z Plan also exposed one of the deepest contradictions of World War II America.
Enemy Aliens
Nisei linguists were indispensable to the American war effort overseas, while Japanese Americans were simultaneously being viewed with suspicion at home.
Top Allied commanders credited these efforts with saving nearly a million lives and shortening the Pacific war by two years. Their language skills helped translate captured documents, interrogate prisoners, monitor communications, and uncover military plans that saved American lives.
In the case of the Z Plan, understanding plain Japanese became a strategic advantage powerful enough to influence the course of the Pacific War itself.
The same country incarcerating many Japanese Americans was depending on them to help win the war.
Courtesy of Harry Fukuhara
Yoshikazu Yamada, one of the top Japanese American Military Intelligence Service linguists attached to ATIS, helped translate the captured “Z Plan” documents in May 1944.
Photo by Toyo Miyatake
Many Japanese American men volunteered for the Military Intelligence Service while their families remained incarcerated behind barbed wire.
Study was hard. Classes ran from 8am to 4:30pm, then again from 7pm to 9pm. But many MIS students stayed up studying, even in the bathrooms after lights out.