"... When imperial powers of the world have been in their ascendancy, and begin their decline, their societies have become increasingly violent. "

I'd question whether that is actually true. Much of the time the violence gets exported. Nineteenth and early 20th century Britain was much less violent than in previous times, or later times, in terms of law-and-order type violence.

The USA as "a warrior culture"? Well, if a warrior culture means one where people actually go out and fight wars against equal enemies, that doesn't really fit. If it means having a fantasy life about fighting as a warrior, true enough. (Parallels with Roman times here fit quite well too. PLenty of gladiatorial combats in Rome, but the actual fighting, in the arenas and in the wars, was mostly done by non-citizens.)