It can take a long time like Rome or it could be short like the third reich. It will be violent though. The “graceful” British decline hasn’t been anywhere close to peaceful. I think the best explanation for this relatively low magnitude of violence is that the British empire never died, it just lost rank. It was supplanted. It still benefits heavily from imperialism, just in a different form and to a lesser degree. The Suez Canal crisis is a great case study on how the British empire came to understand its lost of supremacy and accept its new place in the world hierarchy.

The big difference with America (and the collective Western imperial system) is that should China succeed in dominating the world economic hierarchy, it could be the end of that hierarchy. At least that’s what capitalist leaders are currently thinking will happen. I believe this decline has to be violent because it is the last flail of a system that knows it is dying rather than a country accepting second best. They think if China wins the jig is up basically.