The Romans depended on foreign soldiers, barbarians to man the Legions at the end. A society defended by men with no stake in it, no ties to it. It did not end well for them.
Allowing entire barbarian nations to settle within the empire in return for serving as completely barbarian armies to defend the frontiers was what sunk the empire in the west. In the east where the Anatolians and even the Syrians felt a bond with the empire the classical world continued to operate for another six hundred years.
Like the Austria-Hungary Army in WWI. Many wound up defecting to the other side.