The US had the enormous advantage of starting its life with material resources of which most can only dream, so it couldn't help but achieve some fairly significant success, and as long as things were relatively easy, it generally did. But it never quite managed to pull its head out of its ass. Its material advantages made it so that it generally managed to get by in spite of the fact that it's head was firmly lodged up its own ass, but that also meant that it never learned anything. So it just stayed in a diminishing circle of bad decisions until it reached a point at which smart decisions were necessary, and it revealed itself to be mostly incapable of making them.
And at the moment, it's actually subject to a mass movement that lauds the days of the bad decisions as the good old days, since the people still have their heads too far up their asses and can't recognize the reality that they were always bad decisions, that the prosperity that accompanied them was simply due to the US's enormous material advantages and in spite of, rather than because of, the bad decisions, and that a return to those bad decisions in an era in which those material advantages have been squandered is just going to make things even worse.
Which, granted, is still sort of a "good run" - much smarter people have still failed to do even close to as well, since they were stuck starting out with pretty much nothing but disadvantages.
But one can't help but wonder what could've been had we not had our heads so firmly lodged up our asses...