In the twentieth century, America veered between isolationism and interventionism, but there was a lot of background to each. Effectively, the latter promoted strong economic growth, resulting in a huge growth spurt after WWII, handily illustrating the thirty fourth rule of acquisition: "War is good for business". The new-found confidence after WWII, which resulted in an era of hitherto unseen prosperity, made the US very aggressive towards anything that appeared to threaten that prosperity - such as Communism - so they developed a markedly anti-communist mentality and policed both their own country (the McCarthy era, which ushered in McCarthyism) and the world to make sure that nothing and no one could damage the US recovery.
But it hasn't worked: what it has done is encourage the development of rabid anti-US sentiments across the planet and proved that meddling with the internal affairs of other countries eventually results in even more problems.
If there's a distinctiveness to US foreign policy, I think it's that they see world events and other political systems in black and white; for US policy makers, certainly, there are no shades of grey, which is both curious and understandable. Curious, because so many of the original US settlers were themselves fleeing repression and persecution and understandable for exactly the same reasons. US history (brief as it is) is also seen in those terms: the North and the South; cowboys and Indians; rich and poor; Communism and Capitalism; Republicans and Democrats; and so on. While they see the world in such simplistic terms, I wouldn't hold out much hope for change, however.
