President roosevelt compared war to an epidemic that needed to be actively contained to keep it from spreading.

how did american isolationism and neutrality actually contributed to the spread of war around the world?

Some historians claim that American Isolationism led certain powers (Japan, Germany) to believe they could attack American allies without worrying about the United States getting involved militarily. This philosophy seemed to hold true up until Japan attacked Pearl Harbor and the United States determined to enter WWII instead of just sending supplies to allies.