In February 1942, a small group of members of a top-secret military language school defied orders. They slipped out of their headquarters in San Francisco and snuck toward their destination, a nearby racetrack.

They weren’t there to gamble: They were there to visit their parents, Japanese immigrants who were about to be interned for the duration of the war. These sons of immigrants were American citizens, but because of their parents’ ancestry, they were considered enemies of the United States.

But unlike their parents, they weren’t headed for internment camps. Instead, they were training to be shipped to the Pacific Theater, where they would become one of the United States’ most powerful secret weapons.

Over the course of World War II, Nisei linguists, many of whom were initially forbidden from serving in the military and many of whom spoke little Japanese before the war, became a critical tool in the Pacific Theater. These children of Japanese immigrant (who were known as Issei) translated crucial documents and assisted with interrogations and interpretations, often during tense battles. They served their country while over 100,000 Japanese-Americans and immigrants from Japan were forced from their homes and jobs and into internment camps. But though they helped the Allies win the war, the Nisei linguists’ contributions to the war effort were kept secret until decades later.

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Before World War II, the United States military had invested little in establishing a Japanese-language intelligence corps. Though there was talk of recruiting Nisei to help with intelligence overseas, there weren’t many people to choose from. In the summer of 1941, the military surveyed the Army to determine if there were Japanese speakers who might be able to help in the case of war with Japan, but it found that of the 3,700 Nisei who were already in the army, only a small number were fluent enough in Japanese to serve as intelligence workers.

It was clear the war effort would need Japanese interpreters, and even clearer that so few white Americans spoke Japanese that equipping them with the skills they needed in time for war would be almost impossible. So in November 1941, armed with $2,000 and four Nisei language teachers, the Army began its first Japanese language school in an airport hangar in San Francisco. “Initially orange crates and boxes functioned as desks and chairs,” writes historian Kelli Y. Nakamura, “and, given the short supply of texts, trainees studied from mimeographed sheets.”

Forty-five students graduated from the program in May 1942—a full quarter of the class had failed because the program was so difficult. By then, the Japanese had attacked Pearl Harbor and the United States was gripped with paranoia about Japanese Americans.

Two months earlier, backed by an executive order by President Roosevelt, the United States had begun forcibly relocating people who had 1/16 Japanese ancestry or more, sending them to 10 internment camps around the country. This “evacuation” was accompanied by xenophobia and prejudice, and Nisei who were already in the military were viewed as potential spies and threats, and many were discharged.

Captain Jerry Tobin, infantry recruiting officer, shown swearing in a batch of Nisei youths about to undergo examinations of their ability as interpreters, in written and spoken Japanese, recruited for service in the US Army. Bettmann Archive/Getty Images

In June 1942, just as the graduates of the first language school began their first tours of duty, the War Department declared it would not allow any people with Japanese ancestry to enlist in the military. Because Japanese Americans had been excluded from the Pacific coast, the Army decided to move its school from San Francisco to Minnesota. Now dubbed the Military Intelligence Service Language School, it began training hundreds more Nisei men, many of whom were recruited from internment camps and whose relatives and families were interned for much or all of the war. Eventually, more than 6,000 Nisei men would graduate from the program.

Eventually, Roosevelt lifted the restrictions on people of Japanese descent serving in the military. By then, Nisei men were embedded in units all over the Pacific theater. More than half hailed from Hawaii, where 37 percent of the population were people of Japanese decent (due to concerns that internment would hinder the war effort on the islands, the entire island was placed under martial law instead.) They listened in on communications, translated documents handed over by prisoners and helped interrogate them. And in 1944, they got their most important assignment yet.

The “Z Plan” would never have fallen into American hands if it were not for a fatal plane crash. The plan, which had been devised by Mineichi Koga, the commander-in-chief of the Japanese fleet, intended to devastate the U.S. Pacific Fleet through a decisive battle in the Philippine Sea. It included details on all Japanese naval forces and their assignments—valuable information that was closely sheltered. But when Koga’s plane crashed in a typhoon over the Philippines, he died…and the plans he was carrying with him were lost.

Vice Admiral Mineichi Koga, pictured in 1943, a year before his death in a plane crash. Corbis/Getty Images

As his chief of staff, Shigeru Fukudome, was captured and questioned by American troops, Japan began a frantic search for the documents, burning villages and killing civilians. But unbeknownst to the Japanese, the plans had already made their way into American hands via Filipino guerrillas who had gotten them from Filipino fisherman who had retrieved a wooden box from the water when they rescued Fukudome, who had survived the plane crash.

Soon, the documents were on a submarine, then a plane to Australia. There, they were roughly translated by three white officers. Though it was customary to exclude Nisei translators from working on top-secret documents due to bias and suspicion within the military, this situation was different. Two Nisei translators, Yoshikazu Yamada and George Yamashiro, were given clearance to look at the documents. They checked the other translators’ work and printed out the translation, which was then sent by air to Hawaii. (The document was later retranslated by a Naval officer who updated its Navy-related terminology.)

The documents were critical to of the Battle of the Philippine Sea, which used the plans to deal a decisive defeat to the Japanese navy. “The defeat was a blow from which Japan never recovered,” writes historian James C. McNaughton.

It wasn’t the only time Nisei translators played a crucial role in translating battle plans that found their way into American hands. They worked by flashlight in the middle of the night to translate battle plans that ended up helping American forces win the Battle of Saipan, helped stave off assaults in Burma, and even helped with the war in Europe by translating communications between Japan’s ambassador to Germany and Tokyo. They participated in every major campaign against Japan, often doing much more than translating as they engaged in active combat.

There’s no doubt that Nisei translators helped the United States win the war, and after the war they played a critical part in war crimes trials and the U.S. occupation of Japan. But despite their patriotism in a time of overwhelming public prejudice against Japanese-Americans, their round-the-clock work in a military atmosphere in which they feared being fired upon by their own troops—and the fact that Nisei linguists translated 20.5 million pages during the war—it took decades for their contributions to become known.

Though top military brass commended the Nisei linguists, the wider world did not learn about their contributions until the 1970s, when military documents related to the war began being declassified. In November 2011, the Military Intelligence Service and two Nisei military units were awarded the Congressional Gold Medal. But due to the secret nature of their heroism, their contributions to the United States are still little known.