You seem to think oppress means to kill en masse, it doesn't. There isn't much a civilian can ever do if the army (who are incidentally also American citizens so they'll never in a million years do it) start dropping nuclear bombs on their own cities (also, family and friends). If on the other hand they want to start shooting people at polling stations or lawful protesters then fire arms will definitely provide a deterrent.
where were they when anyone of Japanese descent was bring rounded up?
or when black people where legally segregated?
the government has impressed the American people a fair few times in history usually with broad support