What factors contributed to the united states becoming a world leader

The factors that  contributed to the United States becoming a world leader is: the United States suffering no war damage and having a strong economy

Besides the damage that caused by Japan in pearl harbor, none of Nazi's forces came even close to the American soil and most of the world war II battlefield happened within western Europe Area. 

Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.