As I'm sure you're aware, Japanese Americans living along the Pacific Coast were forcibly relocated to internment camps by Executive Order 9066, signed by FDR on 19 February 1942 (ten weeks after Pearl Harbor). As the war progressed, many of the first- and second-generation Japanese Americans (Issei and Nisei, respectively) volunteered or were drafted for service in the U.S. military. The National Archives has a really good book on this entire period, it is called Personal Justice Denied, it is the source for most of the information in this post.

The majority of Japanese Americans served in the Army's 100th Infantry Battalion, 442nd Regimental Combat Team, and 522nd Field Artillery Battalion. All three units saw action in the European Theater, as Japanese Americans were prohibited from fighting in the Pacific Theater. One Japanese American member of the 100th Infantry Battalion even earned the Congressional Medal of Honor posthumously for sacrificing his life to save his fellow soldiers at Seravezza, Italy. There were at least 5 Japanese American members of the U.S. Army Air Forces.

Personal Justice Denied goes into a lot more detail, and I'd highly encourage you to read it if you are interested in the subject. Here is an excerpt that gives an idea of the various roles Japanese Americans served in:

There were Japanese American medics, mechanics and clerks in the Quartermaster Corps and Nisei women in the Women's Army Corps (WACs). Nisei and Issei served as language instructors, employees in the Army Map Service, and behind the scenes in the Office of Strategic Services (OSS) and Office of War Information (OWI). In the latter groups were primarily younger Issei who had fled Japan after World War I to avoid political persecution. At OWI and OSS, some made broadcasts to Japan, while others wrote propaganda leaflets urging Japanese troops to surrender or pamphlets dropped over Japan to weaken civilian morale.