What prevented earlier u. s. participation in world war ii?

Prior to WW2, America had an isolationist foreign policy. It never joined the League of Nations, and staunchly avoided taking side with any colonialist power.

Anyway, America's priority at the time was to tackle the depression (which saw unemployment rose to 25%). On the whole the American population and their politicians were totally ignorant or blindsided to the frighten events in Germany and Japan, until it was too late for the countries invaded by the Axis aggressors.