After his graduation from college Wai enlisted in the Hawaii National Guard. He was called to active duty before the United States' entrance into World War II. He received a commission as an officer and completed Officer Candidate School in 1941. His commission was rare at a time when few Asian-Americans were allowed to serve in combat leadership roles. He was eventually assigned to the 34th Infantry Regiment of the 24th Infantry Division with the rank of Captain. The 24th Division, based at Schofield Barracks, Oahu, was among the first American units to be involved in the Pacific Theater. They exchanged fire with Japanese aircraft during the December 7, 1941 attack on Pearl Harbor.