What prompted the US to change from isolationism to war?

During the 1930s, the combination of the Great Depression and the memory of tragic losses in World War I contributed to pushing American public opinion and policy toward isolationism. Isolationists advocated non-involvement in European and Asian conflicts and non-entanglement in international politics

Explanation:

For WWII:

There were a couple of factor, one of the most memroable was the bombing on peral harbor by Japan, and the Zimmermann telegram. The Zimmermann telegram was a messeage from the German foreign scretary, Arthur Zimmermann, who tried to encourage Mexico to forge an alliance with Germany and bomb America. This message was then leak to America, and a copy was released to the public on the newspaper, creating a public outcry for war against Germany.