In 1934, Ralph Yempuku's father had a stroke and because of his illness decided to return with the family to Japan. Yempuku, the oldest son, wanted to finish his education in Hawaii and stayed while the rest of his family moved to Japan.
At the University of Hawaii, Yempuku joined the Reserve Officer Training Corps. When Japan bombed Pearl Harbor, the Hawaiian Territorial Guard recruited all University of Hawaii ROTC members to defend the island. When Caucasian soldiers came down to the harbor and saw all of the Japanese-American guards, they became confused and the Hawaii Territorial Guard decided to disarm the Japanese-American volunteers. The Japanese Americans still wanted to participate in the defense of the island and formed their own non-combat volunteer unit. Many Japanese Americans from that group joined the 442nd Infantry Regiment, and Yempuku signed up in May 1943.
While training at Camp Shelby, the Office of Strategic Services (OSS) came to recruit Japanese-American soldiers to work for them. Yempuku signed up and then attended the Military Intelligence Service (MIS) Language School. Unlike other MIS students, he went undercover and studied in separate classrooms. After four months of training, the Army sent him to the China-Burma-India (CBI) theater and then dropped him behind enemy lines in Burma. He commanded 50 to 60 guerrilla soldiers from the Kachin ethnic group. The OSS did not want to let the Japanese know that Japanese Americans fought for them, and neither did Yempuku:
I didn't want Japanese ever to find out that a Yempuku was in the American Army in Burma because they would certainly do something to my parents or to my brothers ... the Japanese are very vindictive people. [Uncommon Courage]
North Burma is densely jungle country. There's only one-way to get from one village to other, just a trail. There's no roads, no cars or anything, just a trail. And whoever controls the trail controls that area. And we tried to control the trails by setting ambushes (to) prevent the Japanese from using that area. [Uncommon Courage]
Unbeknownst to me, Donald, my brother was interpreter for the Japanese general. So I'm looking down, I don't recognize him [Yempuku's brother], you know. But he looked up and saw me. He recognized me. [Uncommon Courage]
If I was fighting against the Germans and Italians, they could forgive me. But fighting against their, you know, my own kind, against the Japanese, this is what hurt them the most. So they were very, very upset.... My brother told me they wanted to harm me. [Uncommon Courage]
They said that they found, when a B-29 came over Japan ... there was a Nisei crewman and he dropped his shoes out of the plane. And in those shoes was a message for the Japanese to ganbatte [persevere and fight hard] and about all these Nisei getting killed as cannon fodder ... they figured that ... I would be dead.... They were shocked, as if they had seen a ghost. [Uncommon Courage]