Civilizations do rise and they do fall, so those that believe that there is absolutely nothing that will ever knock America off of its lofty perch simply don't know their history. Think of great civilizations such as: the Roman Empire, the Greek Empire, Ancient Greece, and the Mayan empire, to name just a few of the most prominent empires the world has ever known, all gone, remembered, but of little import to the world, as it is, today. For America, the last chance that other countries had to stop them from becoming the most powerful nation on earth was its Civil war, in which, the European nations, watched with interest, but did not interfere with, and after America re-united, consolidated, and got back on track, it essentially through its size which placed it on both great oceans of this planet, its industrious people, and its desire to strive to be all that it could be, became the dominant nation entering into the very beginning of the 20th century, a power that it has not relinquished, for well over 100 years.
That said there are many ways and many possibilities that America could fall of which some of these possibilities are as follows:
1. Another civil war, whether it consists of the minorities v. the white majority, the Democrats v. the Republicans, the massive underclass v. the elite majority, of which, all of these could indeed happen.
2. The decay from within, in which the moral compass of America simply erodes to nothing, so that truly anything goes, bringing us constant confusion, uprising, and endless troubles.
3. An economic collapse, in which the monetary system as we presently know it, defaults, causing massive unemployment, massive civil unrest, and massive economic change, all done in a haphazard and disruptive manner.
4. The resentment of the world at large, that since America muscles everyone else, as well as constrains everyone else, but never lets international law preclude it from doing what it so desires, in which the world, unites in its avowed purpose to take down the richest and most powerful nation the world has ever known.
5. A military coup, because the President in conjunction with the House, finally decides that devoting nearly 1 trillion dollars to our national defense, year after year, is literally stealing food and opportunity from the good people of this country, so that an overdue change to such, is actually implemented, forcing the military to either bow down to their Chief Executive, or to seize power from that Chief Executive and declare martial law.
6. An active rebellion and determination from the likes of California, Texas, or New York to secede from the nation, and to become independent of it, cascading to more States doing the same, till the United States simply is no longer united and is no more.
All of the above are possibilities, though nothing much more than that. The real probabilities are shown below, for the earth and this universe are in constant flux, and sudden change can have catastrophic consequences, for instance:
1. America is the bread basket of the world, but due to incredibly colossal volcano eruptionsfrom Yellowstone, as well as the Cascade range, America's weather patterns are disrupted so dramatically, that crop yields drop to a mere fraction of what they previously were, leading to massive starvation, massive civil disruption, and collapse.
2. Earthquakes of unprecedented fervor rock America, to its core, and Oregon, Washington, and California, within just a few days, completely vanish into the Pacific Ocean, in addition, New Madrid, Missouri, in the heartland of America, develops earthquake levels at unheard of levels, swallowing up much of Middle America.
3. An asteroid of a truly impressive dimension strikes the American homeland, wreaking utter destruction, not only at the impact zone, but because of the energy waves so generated this also lays waste to a radius far exceeding the size of the asteroid, basically bringing down the complete edifice of America to the point of no return.
Sure, America is the only superpower, but it isn’t indestructible for it cannot control "acts of God", but it does have the ability to work together with other nations in harmony, respect and brotherly accord.