The United States wanted to make the world safe for democracy. Instead, isolationism made them turn their backs to the Europeans during World War Two. The United States of America had been established as the World's economic power because of its banking and commercial expansion during the 1920's. Even though diplomatic and military presence ha reduced during the same time.
During the inter-war years America had become an isolationist nation. It only became invaded in foreign affairs if its economic interests were threatened. America was the largest producer of manufactured goods and food, had the largest stocks of gold, booming automobile and an extensive domestic market. This allowed the USA not to have to depend on other countries for these things. The standard of living was high and investment capital was readily available.
The world's raw materials were not needed by America. She became economically protective and when the economy began to slump in 1939's this got in the way of any solutions to the problems.
America had become selfish. They didn't have anything to do with International organizations like the League of Nations. America had also become neutral. They refused to take sides on any world affair or any conflicts until 1939. When they took Britain's side during the war.
What caused America to come out of isolation and neutrality was the attack at Pearl Harbor. This caused America to become involved not only in the Pacific war but also the World Was in Europe. The US kept a neutral stance in foreign policies for most of the 1930's. People living in US heard about the crisis that was taking place in the Europe, but most did not feel it affected their lives.