The failure to integrate the barbarian Goth refugees is generally marked as the beginning of the fall - and absolutely changed the trajectory of the empire.

“Failure to integrate” is one way to describe the Goths sacking Rome. But I agree the barbarians did them in at the end. My point was that the Iran fiasco is more like the Battle of the Teutoburg Forest, where Rome backed off after taking a beating. That didn’t mark a transition away from Empire, simply a retreat after a military loss.