Aiso was the Director and head instructor of the Military Intelligence Service Language School, and the highest-ranking Japanese American in the U.S. Army during World War II.

[3] After graduating at the top of his Hollywood High School class in 1926, Aiso spent a year in Japan, studying Japanese at Seijo University in Tokyo.

He returned to the United States after receiving a scholarship to attend Brown University, where he captained the debate team and majored in economics, graduating cum laude and serving as class valedictorian in 1931.

Originally stationed at Fort MacArthur, Aiso was assigned to menial tasks due to discrimination, and was working in the motor pool when his proficiency in the Japanese language was recognized by Fourth Army G-2 officer, Capt.

[4] Recruited by MISLS head Lt Col John Weckerling as an instructor at the school, Aiso, then a Private First Class, couldn't be expected to teach officers, and Japanese Americans were prohibited from being commissioned at the time, so he was to be discharged, transferred to Reserve service, and hired as a War Department Civilian.

[1] Aiso distinguished himself in his role, earning praise from his commanders and his fellow instructors, and was soon appointed the Director of Academic Training, a position normally held by a Lieutenant Colonel.

When the Chief of Army Intelligence, General Clayton Lawrence Bissell, visited the school in 1944, he was outraged that a civilian was placed in command of military personnel, and went back to Washington to demand a direct commission for Aiso to Major.

In this position, he became the highest-ranking Japanese American in the United States Army during the Second World War, eventually separating from active duty with the rank of lieutenant colonel.