Many of the interned Japanese were second or third generation US citizens. It was racism and economic competition, and also pandering to important voter constituences, pretending to have something to do with military circumstances. Check out https://en.wikipedia.org/wiki/Internment_of_Japanese_America...

This was done despite multiple reports of the military officers who studied the problem and stated that the Japanese pose no threat, are often US patriots, and need not be isolated.

Imagine Russia interning everyone of Ukrainian descent up to third generation because of the war, and everybody be like "Well, fine, they are doing a reasonable thing, let them go on".