There were three distinct phases in American foreign policy during this time.
First, after WWI, the US was interested in trying to prevent further wars. It wanted to do this through treaties. For example, the US was involved in the Washington Naval Conference in which treaties were made to prevent naval arms races. This was one of the causes of WWI and the US wanted to avoid a repetition of that war.
Second, as the Depression began, the US moved towards isolationism. During the 1930s, the country passed a series of Neutrality Acts. These acts were meant to prevent the US from trading with any country that was involved in a war. This would prevent the US from getting dragged into wars through trade, which was what had happened to the country in WWI.
Finally, at the tail end of the Depression, the US was working on getting involved in WWII. President Roosevelt wanted to get the US involved in the war because he felt Nazi Germany was a real threat. Eventually, before the end of the 1930s, the US was aiding the Allies even though it was not officially involved in the war.