By the late 19th century it was clear to some that the United States was going to be a major contender in world politics, because it had an incredible resource base, relatively stable politics (after the Civil War), and a strong industrial base. Generally historians attribute its actual rise to superpower status to the two world wars, which both encouraged American ambitions abroad and devastated the traditional European and Asian sources of wealth and productivity. World War II in particular left a world in which the United States was the only major power unscathed by the war economically, with a military force that could be deployed on a global scale, and an ambition towards global projection of political and economic power (in the name of security against the USSR) that was unprecedented (the isolationist instincts of the past were largely quelled). Many of these trends began in the interwar period but were stalled by the Great Depression; all of these trends have historical antecedents, and like all things, this sort of big change is multi-causal, but I think saying "World War II" is as good a single-cause answer as you are going to find from historians.