Canada

The death throes will cause a lot of collateral damage worldwide, though.

It's not over 'til it's over. We're not out of the woods yet.

Yes and no. The decline already started before Trump's first term. The peak was the 90s or early 2000s, but even then the conditions that caused the decline were already in place. They had hollowed out their industrial base and offshored manufacturing in favour of a financialized economy, and it worked very well for a time. It restored profitability and disciplined labour, effectively neutering unions. America's technological advantage carried it through the 90s and 00s, and its Cold War era military remained unchallenged for a long time.

However, I emphasize that it was the Cold War era military. All that military power came from manufacturing, but that hollowing out of their manufacturing base also hollowed out arms manufacturing. The defence contractors merged and conglomerated, and they became even more profitable, but their actual material output has been dropping precipitously as they focused on high margin low production rate technological wonder-weapons. They made oodles of money, but delivered little. Still, with the leftover Cold War military as the backbone and the cutting-edge modern equipment as the teeth, they were able to dominate smaller states and remain a major threat to the other large military powers.

However, the rot really started to become apparent this decade. The Ukraine war revealed that Russia's leftover Soviet factories could outproduce all of NATO in terms of artillery shells, one of the most basic pieces of war materiel. The Houthi blockade of the Red Sea drained naval interceptors at an unsustainable rate, being used up against relatively cheap missiles and drones. In manufacturing terms, America already couldn't keep up.

Then, in walks Trump 2: the Revenge Term. To his (very limited) credit, he did understand on some level that America had been deindustrialized - he had repeatedly said that "if you don't have steel, you don't have a country" - and his tariff spree last year was aimed at bringing back domestic industry. It was too little, too late of course, and only harmed America's global image without significantly restoring domestic manufacturing. He also seems to have either forgotten this, or never really understood the ramifications of deindustrialization for military power, or convinced himself that he completely restored american manfacturing. Probably a combination of the three given that he's senile.

He had also absorbed the right wing cultural theory of American decline, that they had gone woke and therefore gone broke, and all they needed to do was violently reassert themselves on the world stage in order to reclaim their old glory. This analysis is of course completely at odds with material reality (that America no longer has the manufacturing power to maintail a global military empire), but Trump is stupid, senile, and very suggestible, so he managed to get talked into going to war with Iran, despite knowing that American manufacturing had been hollowed out.

Now they've exhausted a large proportion of their precision-guided standoff weapons (harpoons, JASSMs, etc.), their interceptor missiles (THAADs, Patriots), they had an aircraft carrier almost burn down due to a laundry fire, and another carrier serving what looks like boiled shoe leather since they also hollowed out their logistics.

This wasn't all Trump's doing, he just came in as the right man at the right time to completely fuck up the US empire. They probably had a few decades left of global hegemony, but in trying to reassert themselves when they no longer had the juice to fight a middle power like Iran (let alone China), they've drastically accelerated their decline.