After World War I, the US wanted to remain out of foreign affairs. The US managed to do this for the most part, passing different laws that helped them stay neutral as long as possible. A perfect example of this would be the three Neutrality laws passed by the US Congress in the 1930's. These stated that America would not: 1) Trade with countries involved in wars. 2) Enter waters that were considered hostile. 3) Have citizens on board ships that might be considered hostile.
This would last throughout the entire 1930's. It is not until the attacking of Pearl Harbor that the US officially enters World War II in 1941.