This discussion is so wide ranging that I feel out of my depth in formulating anything other than a moan. So in the spirit of moaning some thoughts:

The political power of an entity, as a relative quantity, varies not only as an absolute, but in relation to other actors. US's power has diminished partly because the power of other Powers especially in their own neighborhoods has increased since, say WWII, when the US stood alone as an intact power.

Still, the military and economic power of the US has grown in absolute terms. Just less market share.

Leaving aside the economic dimensions of neo-imperialism, I wonder if a significant part of the military uniqueness of the US in both sheer size and sophistication, derives from the dream of being the sole nuclear (and generalized to WMD of all sorts) power in the world. The various non-proliferation regimes all depend on a unique role of the US.

Bush attempted to extend this monopoly by stressing interdiction of WMD materials as they moved about the world to the practical exclusion of negotiation to contain them. While many nations signed on to the initiative, for all practical purposes it was an American as say the Iraq war. No other power had the reach and sophistication to even attempt it.

But succeeded probably even less well than the war in Iraq.

Which is not to say that a negotiation strategy would have worked better. Diplomacy has obvious limits.

Honestly, I'm having trouble following my own line of thought here, so I'll leave it hanging unresolved for now, maybe to pick it up later, hoping that this contains something worthwhile.