Pretty much private mercenaries that work outside of the usual army structure as "private contractors". They're usually the ones the US contracts to do the worst atrocities, as that gives the government a thin veneer of plausible deniability because they were behaving "independently". The US also does its best to make sure they never face any legal consequences for their war crimes.

Also worth pointing out that, due to this "contractor" relationship, they never count towards official casualty figures. For example, if Iran were to kill 50k of them (I'm of course exaggerating to make a point), they wouldn't count towards US casualty figures, so it's also a way for the government to downplay the effects of foreign intervention to the general public.