@Theaetetus,
Many nations have, by turns, been "the greatest country on the face of the planet." Some, such as Rome, could lay claim to that title for many centuries. Others including the Spanish, the Dutch and the British held the honour for much briefer interludes.
Until just a hundred years ago, most Britons would boast their homeland to be the greatest country on earth and if you looked at a map of the British Empire it would be very hard to argue against them. Britain fell into decline during WWI, a process that was pretty much complete by the end of the sequel, WWII.
India and China are ascendant - again. Yes, again. We Westerners view the world in the European tradition going back to Rome and rarely consider how relatively weak and backward we were for so long compared to China and India. We were able to couple our technological advancements and occasional lapses of scruples with disunity and a good measure of chaos in those nations to effectively take over and dominate but, as they say, all good things must end.
But relax. Stepping down from the top perch does not mean one has to fall to the bottom of the birdcage. Not at all. America will remain "a" dominant nation but there will no longer be a unipolar world. This is a very critical moment for America as, to a considerable extent, it will define itself in this new and difficult age we're entering. If, however, it remains in some blurry, hyper-patriotic state of denial it will allow some very important opportunities to slip between its fingers.
Look at it this way. The world in which America, like Britain, the Netherlands and Spain before it, rose to supremacy is coming to an end. America had just truly cemented the mantle of superpower when I was born. That was a world of just two billion people. In the era of American hegemony that population more than tripled to what is now approaching seven billion souls. The United States can take credit for introducing to the world science and technology that allowed this growth, even if that growth has become utterly unsustainable.
Now we're coming to see this global economic and technological miracle as very much a deck of cards that is astonishingly fragile and bound to collapse. When you have a planet that has a carrying capacity of roughly four billion people and you twist it here and there to make room for seven billion and, eventually, up to nine billion, you haven't really left much for your inevitable successor to inherit.
All things considered, this may be a most fortuitous time for the United States to step out of the unipolar limelight. Count your blessings.