For much of world history, global politics has been multipolar, with typically several powers or political rivals competing with one another. That the U.S. became *the* power happened by circumstance, mainly owing to the fact that Europe was much poorer after WWII than before it. Up until the 1920s-30s, the center of global financial power was London, which is still to this day in the top two or three financial centers worldwide. After 1945, though, it was indisputably New York.
The U.S. is still *the* power, but what most people don't realize is that we've lost a *lot* of our financial dominance since 2000, and it's a trend that will likely continue. Although the U.S. is still the largest economy in terms of GDP, China is catching up and will eventually supersede us sometime in the next 5-10 years, barring unforeseen circumstances. But if you look at the collective purchasing power of any given country, China is actually already ahead of the United States, and that trend is likely going to continue for the rest of our lives.
Something that puts geopolitical and global financial power into better perspective is global trade on a bilateral basis. In other words, look at the number of countries that call the U.S. their top trading partner. In 2000, about 75% - 80% of the world's nations called us their number one, most important trading partner. Now? Only about a third of the world's countries do. Some of that has gone to regional powers like the European Union, but China has, far and away, stripped the U.S. of its influence more than any other. A strong majority of countries now regard China as their top bilateral trading partner.
That is why Trump's America First foreign and trade policy, if fully implemented, would be a foreign policy disaster from which the U.S. would have a very hard time recovering. Because China and Russia would use our isolationism to drive a wedge between North America and the EU, country by country, one by one. And one by one, China, Russia, and other authoritarian powers would seek to isolate countries, manipulate their domestic politics, and peel them away one by one. Look at China's, South Asia's, Africa's & Mid-East's population and resources, and look at that of Europe and North America. Who do you think wins that competition?