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.