If you ask people why the USA has, for a long time, been the world's most powerful nation, you're bound to get a lot of vapid answers about "can-do spirit" and "the American dream" and other things that don't even try to answer the question. At best, they push the question back one step: so why did the USA get that spirit more than everyone else? At worst, they don't even attempt to answer the question. A real answer doesn't just say why the USA did what it did, but why no one else did quite as much.
So why the USA more than anyone else? Naturally there's a hundred contributing factors but I think there are two that stand out more than any other. They're the things that got the momentum going to the point where it became self-reinforcing and self-sustaining.
Resources: At the time when the colonies were forming into a new nation, most everywhere else on the surface of the Earth was either at least a little hostile, or unpopulated, or had been stripped of much of its resources. The lands that became the United States were an unprecedented trove of natural resources. Vast areas of fertile, welcoming land that could provide crops to support an amazing amount of growth. Lots of land for people to live in, and very little of it was jungle, mountain, desert, tundra, or otherwise very harsh. Tremendous amounts of timber -- it's easy to miss how crucial this was, but Europe had been stripped of most of its timber long before, and wood was the main building material for almost all industry for a long time. Plenty of game. Mineral resources undreamed of by Europe: metals, coal, and everything else a growing country needs. All of it pretty much just there for the taking.
We always imagine the frontier was harsh, and certainly if you dropped me into 1804 I wouldn't last long outside the cities, but it was harsh in a totally different way from, say, Canada, or Australia, or Brazil. There was a living to be made; and if you engaged in unsustainable practices, there was always another forty acres west of you that could be exploited. (That didn't run out until the 19th century, giving the country centuries of time to spend profligately and build up a nest egg.)
The nation started out rich. All that wealth was more than just a big pile of capital in its pocket. It encouraged industry and made risky ventures and large, complex projects (like the Erie Canal) viable. It attracted the sort of people who would take on projects like that. This is the genesis of that "can-do spirit," I think: things were comparatively easy enough that people could do stuff, and thus, people who did stuff came, and were encouraged to keep thinking they could do more stuff.
Inventors: When we think about the founding of the nation, we always think about the politics. Freedom of religion, checks and balances, stuff like that. Those are certainly important stuff. But there's one important thing that we don't think as much about. At least two of the three most important Founding Fathers embodied not just the enterprising can-do spirit but also the Renaissance Man's fascination with discovery and invention. Thomas Jefferson remained a student of science and an inventor his whole life, and left behind a considerable legacy of both inventions and the academic support for fostering the sciences. Benjamin Franklin is of course famous for being a scientist and inventor almost as much as he was a politician.
And while Adams and Hamilton were making important decisions about the role of government in the life of the citizen, and Franklin and Jefferson were involved in that too, they get a lot less credit for helping to promote the idea of scientific inquiry and industry. It's not just things like helping to set up the idea that led to the patent and copyright systems (explicitly intended to foster further invention and creativity by protecting the rights of creators, while also allowing their words to spur others into doing more), or standing as a good example. They imbued the very idea of an appreciation for scientific inquiry, for examining the world around them, for industry and determination, right into the fabric of early American culture.
When you think about it, it's really remarkable, even in the era of the Renaissance Man, that two of the most important politicians that happened to be in the center of everything that shaped the newborn nation, should also happen to be such excellent examples of the scientist and inventor. The Continental Congress was full of people from many disciplines. Many of them were businessmen, or economists, or historians, or tacticians. Some are really essentially politicians and nothing more. If the dice had fallen another way, maybe the most important people would have happened to be Caeser Rodney and John Hancock instead of Thomas Jefferson and Benjamin Franklin, and even the tiniest differences in their attitudes, magnified by repetition over decades and centuries, might have shaped the country in a very different way.
Once these things got America's can-do spirit, and the American dream, started, it becomes self-sustaining. The plentiful resources provided enough to allow projects to be undertaken which would provide even more resources (a fertile farmscape supports a large mine; a plentiful supply of timber provides both the wood needed to build a canal and the justification to make it worth its cost; a great supply of coal is needed to build a dam that will help provide energy when coal isn't enough anymore). A culture that believes it can do big things will take risks, and then reward people who do big things, and thus encourage more people to do the same. And so on.
If these are indeed key factors in the shaping of America's preeminence, then they also show why that preeminence is fading fast. When westwards expansion reached the Pacific, there were still new frontiers in all the land that didn't get exploited on the way west, but by now, our wealth of available, exploitable natural resources is scarcely more than an average nation of our size and comes nowhere near to meeting our needs, which have continued to grow on the same runaway expansion curve that used to be no problem because there was always more land to exploit. And in the last century, even as our preeminence in technology crested with the birth of the computer age, we've succumbed more and more to a wave of illogic and a repudiation of science, and allowed our place at the forefront of science and technology to progressively erode.
We still act like we are the biggest and best, and we consume as if we still had an endless supply of everything. Little by little we notice that we can't continue our reckless expansion, because the things that used to make it work are gone, but we don't react by becoming what we can afford to be, in a sensible way. We don't change the habits of excess. We keep the same voracious appetite for resources that we had back when there was plenty to sustain it, and when science reveals to us that there isn't anything left to sustain it, that we're running out, we demonize science -- even though it's the only thing that could have found us a new frontier, a new way to get back the growth that could sustain (for a while) our unsustainable appetites. We want to have our cake and eat it too. And as we keep pinning ourselves between these impossibly opposed problems, we let slip away the only opportunity to preserve part of our place as a superpower, because we won't settle for the part we can keep, so we lose it all.