Once an empire has subjugated it's territory by force, there is often a time of "peace," enforced through domination and the elimination of any threats to the empire's authority. Of course, empires rise and fall, so it is inevitable that that peace will end, as the empire retracts to deal with internal problems. This generally leaves the subjugated territories in a position of chaos and instability, as the existing power structures no longer have foreign backing to prop them up.

This is why, even the most moral and well-intended empire (if such a thing could exist) is still problematic on a fundamental, systemic level. I actually made a post about this concept a few years ago. Countries and governments need to be able to stand on their own two feet, backed more by domestic popular support than foreign powers. Of course, there are sometimes pragmatic compromises that have to be made, and no country can be fully isolated from "The Great Game." But I would hardly consider chaos and conflict coming from the retraction of the US empire as a reason to think that empire was a good thing. To me, it just shows that the geopolitical order has been dependent a single point of failure.

I think that's their point, that a segment of the left thought pax americana was pure propaganda and it wasn't. It wasn't benevolent but for years it did keep global conflict tamped down.