While he was working at the steel company, Warren began attending night school at a junior college in Kansas City, then enlisted in the military in the Army Specialized Training Program. As an Army trainee he was enrolled in an accelerated engineering program at Kansas State University. After doing well academically there, he was accepted into a military training program in Japanese language and area studies that was being conducted at Yale University. He completed that program shortly after his 18th birthday, having spent four-quarters at Yale. As an 18-year-old in the summer of 1945, he began basic training at Camp Roberts in California. His military training was cut short after Japan's surrender ended the war. The Army then sent him to the University of Minnesota to continue his studies of Japan and its language. After three months in Minnesota, he was sent to Japan, where he spent the final year of his U.S. Army service.