The story is the same every time: some nation, due to a confluence of lucky circumstances, becomes powerful—much more powerful than the rest—and, for a time, is dominant. But the lucky circumstances, which often amount to no more than a few advantageous quirks of geology, be it Welsh coal or West Texas oil, in due course come to an end. In the meantime, the erstwhile superpower becomes corrupted by its own power.
As the endgame approaches, those still nominally in charge of the collapsing empire resort to all sorts of desperate measures—all except one: they will refuse to ever consider the fact that their imperial superpower is at an end, and that they should change their ways accordingly. George Orwell once offered an excellent explanation for this phenomenon: as the imperial end-game approaches, it becomes a matter of imperial self-preservation to breed a special-purpose ruling class—one that is incapable of understanding that the end-game is approaching. Because, you see, if they had an inkling of what’s going on, they wouldn’t take their jobs seriously enough to keep the game going for as long as possible.
The approaching imperial collapse can be seen in the ever worsening results the empire gets for its imperial efforts. After World War II, the US was able to do a respectable job helping to rebuild Germany, along with the rest of western Europe. Japan also did rather well under US tutelage, as did South Korea after the end of fighting on the Korean peninsula. With Vietnam, Laos and Cambodia, all of which were badly damaged by the US, the results were significantly worse: Vietnam was an outright defeat, Cambodia lived through a period of genocide, while amazingly resilient Laos—the most heavily bombed country on the planet—recovered on its own.