It's very obvious that 
is circling the drain now, but clearly this didn't just happen in a vacuum. It must've started crumbling at some point... but when?
Some people might say it started after 9/11, others might say it goes back to the Raegan administration. A few might even say it started after losing the Vietnam War, or when they went off the gold standard. Or maybe even earlier...
What do you think? 
The big glaring difference is that the US don't have a US to hand things off too. From a certain perspective, China has done what we did to the Brits; studied what worked and what didn't, implementing accordingly. They effectively have a Keynesian-style New Deal economy, only without the liberal backslide into Gilded Age conditions.
But we're not going to hand things off to China. Er, well, we did, happily. With dollar-signs in our eyes and a KA-ching! noise and everything. But we're not going to accept the consequences of that. Maybe instead of accepting the laughable title of "#2 most powerful country in the world" like the Brits did, we'll just be doomed to a forever-Cold War. Completely dependent on the economy across the
PondPacific, but stubbornly refusing to acknowledge it.This is extremely stupid and therefore the most likely scenario.