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.