Question

Why was isolationism strong in the United States in the early 1930s?

Answer 1

There were lingering (and justified) doubts about whether the US should have gotten involved in World War I.

Explanation:

Between the failing Weimar Republic and the growing popularity and influence of Hitler's National Socialists, obviously something was on the verge of happening in Europe.

To most Americans circa 1933, World War I was still a pretty recent memory and despite the platitudes of the "Saving the World for Democracy" speeches, few Americans could remember any compelling reasons for America's involvement in that war. "Preserving our commitment to curtail Hapsburg encroachments" sure wasn't worth Uncle Frank's missing leg, and those crazy Europeans had a pointless war every thirty years or so with or without American involvement. Another coming war did not look to be very different.

In retrospect, it was very different indeed, but that wasn't obvious to most Americans in 1933.