Well, before WWII it wasn't involved enough.
Anyway, I feel that after WWII America was a superpower, so it was, in a way, obligated to be involved in everyone else's business. Also, America was far from a good country at a time, but it was way better than the communist countries, who were really fascist.
Also, as historian James Stokesbury outs it, after WWII there were really only two powerful nations, USA and USSR, everyone else were basically satellites of these two nations. So therefor, the UK, France, West Germany, and all the others could not and would not act without the USA, so therefor, in things like the Korean War, they had to get involved, or else enrage their allies (satellites.)