There were various reasons why the United States decided to follow a policy of isolation during the 1930's.
First off, the Great Depression pushed people away from foreign affairs as they wanted to resolve internal affairs first.
Second off, the number of soldiers lost in World War I kept citizens of the USA weary of foreign countries.
Because of World War I, many citizens that were advocating for isolationism pushed the country to stay out of World War II and affairs that they had no business in (at least until Germany sunk the Lusitania).
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