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? 
I'm gonna answer the opposite of this question, cus there's already some good answers here already; "When does decline stop, when does the structure die and some new historical arrangement emerge?"
The answer is, it might not. Look at our preceding hegemon, the British Empire. They still have that monarchy! And the same parliamentary government, with a special extra house just for nobility. The off-shoring of production, a steadily declining living standards, and a growing irrelevancy on the world stage may be the most clear markers of a chapter titled The End of Empire in a history book... but, without actual effort and organizing, all the organs and chambers of empire just... keep existing. They have momentum, but only just -- everyone involved thinks it's easier to keep it up than start over, so it limps along. It's weird as fuck and mildly horrifying.
Yeah I wonder if that's the actual direction the US will go. It won't be like the empires that ceased to exist in the wake of WWI but more like the british empire that gradually becomes less and less relevant.
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.
I think the UK is a little different than where America is heading because their colony inherited all their power. You could easily view America as an extension of the British Empire. I wouldn't be surprised if its seen that way in the long view of history. I don't think China is going to be quite as chummy and enabling of America as is, the way the Americans were with the British.
I don't think enough is being said that a key difference between the US and the UK is it's geography. It's straddles North America, it has multiple great ports on two oceans, a massive river and lake system in it's center and north, an abundance of resources, and no challenger to it's strength in this hemisphere, though Brazil may approach it. Yes, the manufacturing has gone but with the right plan there is more than enough left in the US to return to that point, if the will were there. What I'm saying is, the US may find itself decline into a multi-polar world, but it will last on as one of those poles for a very long time. Really the only thing at the moment I see changing that is internal strife ripping it apart. And climate change of course, but even then it's so large that various parts will survive. This isn't too sing it's praises but to illustrate that it is a force to be reckoned with, we can hope it will be a paper tiger across the pacific but on it's continent it will have a great deal of muscle for some time. At the very least it's structures will rot but the culture it has created will mean what inherits them could be terrifying even compared to the it's current form. All that is to say it's our duty here to force it's attention away from the international theater to allow movements to grow, and attempt to counter it's evils within, so that at the very least it can be directed towards building a less murderous society than the one it is today. But again, the US landmass is far too large and diverse for me to see it truly slip from power like those of Europe. Decline in the America of "yesterday?" As a force to be reckoned with globally? Not for a long ass time
I'm just guessing, but I think it will get balkanized at some point and some them will turn to a progressive direction, maybe even socialist if the conditions allow it. It's too big a country for it to stay like that after collapsing in its imperial position, and it being divided into semi-autonomous states with their own laws makes balkanization all the more easy.
People keep talking about balkanization, but I don’t think that’s going to happen. A Balkanized America is probably one of the most unstable things to happen in human history and I think Russia and China would prevent that from happening one way or another. Breaking America up is essentially just increasing the number of nuclear powers in the world which for obvious reasons would be hella fucked.
I don't think they have anywhere close enough military might and political will to keep a balkanizing US together. You could have a repeat of the Eight Nation Alliance where an alliance of multiple nations decide to hold the US together, but that's just balkinzation in another name since those alliance of nations will most likely partition the US into multiple parts with each nation administrating a particular partition. This was what happened to parts of the Ottoman Empire after WWI and Germany after WWII.
Partitioning would work I think, but the main point is the US breaking up independently is not really on the table for the rest of the world. It’s too unstable for other nations to do nothing. Imagine 4 or 5 new Israel’s all threatening to annihilate not only one another, but also any one of old Americas enemies? It’s not going to happen imho
I think this is pretty much exactly what will happen, yeah. Whimper, not a bang, etcetera. The impending climate complicates this a bit, actually...but left to its own devices I think this is exactly what a failed US would look like.