The US is an empire. And every empire in history has undergone a period of expansion, stasis, decline and then implosion.
We are currently in the period of decline.
It's an empire we inherited from European powers throughout the 19th and 20th centuries. Indeed, one could argue that Europe itself is a protectorate of the US empire.
The internal divisions and political strife, the insurmountable debt, the increasing centralization of power in government, the unchecked corruption and secrecy, the endless warfare, are all symptoms of the decline of empire.
Americans often feel like there are enemies and adversaries everywhere, which is why they mount no serious objections to profligate spending on the military. Yet what they perceive to be enemies and adversaries are really just countries trying to assert their independence from the US empire. Countries like Russia, Iran, and North Korea are no real threat to America. They have no intention of attacking our country or violating our trade and travel rights around the world.
Of course, virtually everyone who reads this will disagree with it in some way. But that's another symptom of imperial decline: Mass denial. The idea of the "exceptional" and "indispensable" nation (could anything be more overtly imperial?) is simply too ingrained in the minds of Americans to accept the fact that their prestige and greatness is waning. The British and the Romans were possessed of similar delusions near the ends of their great empires.
This isn't to say the US empire is going to implode anytime soon. More than likely, it will continue to limp along for at least a few more decades before the signs of its inevitable implosion really start to manifest themselves. However, what seems exceedingly clear (to me, at least) is that the empire has entered the unmistakable period of decline.