Wai enlisted in the Hawaii National Guard in October 1940 and was called into active duty before the United States' entrance into World War II. After completing Officer Candidate School at Fort Benning, Georgia, he received a commission as a second lieutenant on September 27, 1941. His commission was rare at a time when few Asian Americans were allowed to serve in combat leadership roles. He was eventually assigned as an intelligence officer with Headquarters Company, 34th Infantry Regiment, 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, exchanging fire with Japanese aircraft during the December 7, 1941 attack on Pearl Harbor.
