Why is the USA the world's strongest country?

The USA is the world's strongest country because of its military force.

And it is the most advanced military force because it has the most advanced fundamental science.

And it has the most advanced fundamental science because America inherited it from Germany which had been devastated.

Germany was the most advanced nation since about 800 when Holy Roman Empire was formed.

And in 1930s most German scientists just moved to the USA, similarly to how Roman scientists moved to Germany in 9th century.

At the end of World War II not only the British and French colonial empires ended, but also France and Britain were dependent and had a huge economical debt towards USA.

During the decades that followed, the United States offered good opportunities to skilled professionals in all the world, and also to not skilled ones. That includes researchers, engineers, doctors, and many other professions that did not have the possibility to emerge in their own country.

Even in 2005 it had the highest immigrant population.

US also has the highest military spending, fact that allows to influence the politics of other countries from which they buy raw materials. This made many people use the term American Empire.

Later it was the center of the transformation of economy due to new informatic technologies, internet and e-commerce.

In the latest decades US companies started outsourcing industrial production to China and services, like call centres and computer software development, to India.

The United States is one of the five largest countries in the world by area. The others are Russia, Canada, China, and Brazil. Of these five, the United States has the largest amount of land in temperate, agriculturally and industrially suitable climates. Russia and Canada are (mostly) too far north, China and Brazil have much larger proportions of deserts or jungles.

The United States is also one of the five largest countries in the world by population. The others include China, India, Indonesia and Pakistan. Brazil and Russia are sixth and seventh. It is for these reasons, that the "BRIC" countries are now important, as potential challengers to the U.S.

Of these, only China can beat the U.S. in both population and land area (India also has a respectable, but not matching land area). For now, at least, the quality of the U.S. population is better; richer, better educated, more entrepreneurial, etc. America is an immigrant country that has (generally) attracted the "best and brightest" from abroad. China and Russia, have been "closed" countries that have repelled foreign ideas, putting them at a disadvantage vis-a-vis the U.S. Brazil seems like a smaller version of "America," and India, an English-speaking country, has more (but poorer) people on a smaller land area.

The world is now moving toward a "convergence." Given its natural advantages, America will long be at least a "contender." It's not clear how long we'll remain number one.

Geoffrey Blainy, one of Australia's greatest historians, dedicated about 2 pages or so to a chapter entitled The Paradox of Isolation. He said that places like the USA or Australia advance slowly in the beginning because they are isolated: they don't trade with anyone, they don't share inovation with anyone, they don't even have the need to innovate, because innovation is only sparked when it is necessary. However, at the same time, Blainy also said that isolation, though in the beginning can be hampering, is helpful in the long run, because an isolated nation cannot be attacked, just like Great Britain and the Channel.

What I am trying to get at is that America's isolation was its salvation. In both world wars, America was able and willing to stay neutral for a long long time, and as a result, spent less and earned more on the wars than Europe did. Another example is Great Britain. During the 1800s, Britain persued isolationist policies, and was the greatest empire on earth. By WW1, Great Britain was unable to persue isolation; Germany was building up its navy, and Britain was unable to maintain the two-power standard and was on the decline.

Nowadays, America's isolation is not guaranteed. America has so many obligations around the world that if there is a war, America will be involved. Nor does the USA have isolation from economic problems. Some might even venture to say that the USA is at the beginning of its decline.

Based on the evidence, it seems that isolation is one of the many building factor of a great nation; America and Britain both succeeded because of isolation, and both seem to or have fallen because of loss of isolation.

GDP per capita growth before and after WW2: Note the sudden rise at around 1940.