Well, hold on, Tatsua, I think you're reading it too literally. He was probably forced to leave Japan to escape some specific threat (human slayers, most likely), and then over time forged documents and created a false past in America, allowing him to be drafted. 

Also, Japanese/American relations didn't go sour [i]until[/i] WW2, and it seems he was there quite a while before then. It's likely his first war was WWI.

EDIT: In addition, there were a substantial number of Japanese American troops fighting the Germans... In WW2. It isn't far-fetched at all.