You might read up on the Praetorian Guard. Maybe it was the Varangian Guard? Forces consisting of non-natives are notorious for treachery, mutiny, and revolt. That has always been a problem. It is why Rome grew from nothing with a citizen army, and fell to it's own troops comprised of Goths, although Theodoric simply cut out the corrupt garbage and kept Rome going for decades until his death.

I do get your point, that foreign mercenary forces are somewhat more willing to trample the natives. There's limits to that too, and this is alarming to those soldiers that join the military forces with a promise of citizenship. They are not unaware they are destroying the value of that citizenship when they're doing it.