Answer:
The rise of nationalism
The desire for national self-rule
The desire to unify territories
It depends on which war you're referring to. I feel that the answer you're looking for is the radio, though.
Most isolationists felt that there was no need for americans to feel threatened by developments in Europe and Asia because the vast pacific and atlantic oceans insulated the country from troubles in those regions, and the United States had formed friendly alliances with all the other nations in the Western hemisphere.
