How would you characterize American foreign policy in the 1920s and 1930s?
I would say that American foreign policy in the 1920s was rather different from that of the 1930s. In the 1920s, the US undertook diplomatic endeavors that were meant to make another war impossible or unlikely. In the 1930s, the US largely gave up on this approach and turned to isolationism.
In the 1920s, the US tried to use diplomacy to reduce the chance of another major war happening. The two main undertakings that are associated with this effort are the Kellogg-Briand Pact of 1928 and the Washington Naval Conference of 1921-22. At the naval conference, the US and other nations set limits on the size of the navies that various countries could have. The point of this conference was to prevent another arms race of the sort that helped to bring about WWI. By setting the sizes of the various navies, they would ensure that no country had reason to fear the threat of a naval buildup from other countries. In the Kellogg-Briand Pact, countries joined together to try to outlaw aggressive war. This was, of course, an attempt to prevent wars from breaking out. In the 1920s, then, the US engaged in diplomatic efforts to prevent war.
By the 1930s, the US retreated into isolationism. The most important American foreign policies of this decade are those found in the Neutrality Acts that Congress passed. These acts specified various ways in which to prevent the US from getting pulled into another war as it had been pulled into WWI. For example, the first neutrality act banned the sale of military supplies to countries that were at war. The idea was to prevent the US from getting entangled in the affairs of warring countries and, thereby, getting pulled into the war.
Thus, we can see that US foreign policy in the 1920s was more internationalist and engaged while the policies from the 1930s were more isolationist.