The modern United States is, inarguably, the most powerful country in the world. It leads the entire Western civilization militarily and economically; whether it leads responsibly is an entirely different topic. There is no doubt that it does lead the world, as well as wield incredible influence over every other country.
The United States, however, is only the most advanced country out of the Western civilizations we lead, one that completely dominates the Earth.
The countries which comprise Western civilization far and away outstrip any other nation in health, wealth and, consequently, creature comforts. The countries which are not elements of Western civilization are either emulating it or have done so. China, for example, has a history longer than any country in the West, and yet it was dominated two centuries ago by Western powers and has only recently become an economic rival to the United States.
Why has Western civilization so dominated the world?
Aside from a confluence of environmental variables, the West possesses one key element no other civilization was able to discover and formally set down: science.
From the Greeks' first attempts to describe the world to Newton's formulation of differential and integral calculus and his laws of gravitation and motion, from Darwin's proposal of natural selection and descent with modification to Einstein's theories of relativity to the diverse, modern fields of science and engineering, the rigorous pursuit of scientific descriptions has single-handedly produced the affluence and wealth of Western society.
To understand how science has created our greatness, both military and economic, we must first know and understand what science is.
Succinctly, science is the body of knowledge which we've gained through applying the scientific method, our way of looking at the world and creating useful descriptions of what we see around us. The scientific method and its philosophy - methodological naturalism - permit us to conclude which descriptions of the natural world are more accurate and useful than others. The motivating factor is practicality: How well can we describe the universe in a manner which we can predict and test?
We of Western civilization, with these natural explanations of natural phenomena - the laws of thermodynamics, the theory of evolution, the theories of relativity, quantum mechanics, nuclear chemistry and many, many other useful and accurate descriptions - are able to wield unprecedented control over the world around us.
We are able to build internal combustion engines, combat disease, pinpoint any location on the face of the earth with an accuracy measured in meters, build computers which can calculate at unimaginable speeds, and cram enough power to destroy a city into a metal shell a few meters wide, to name a few feats - and all of this is possible because of methodological naturalism.
We are able to dominate militarily because we understand how to create weapons which are far more devastating than those we used hundreds of thousands of years ago. We dominate economically because, using science's conclusions, we can create more and more products to market, and market them efficiently, which, in turn, creates more and more wealth in society.
We owe all of our wealth, all of our luxury, all of our peace and prosperity to science and its descriptions of the world in which we live.