The Johnson Act (1934) and the Neutrality Acts (1935) effectively prevented economic or military aid to any country involved in the European disputes that were to escalate into World War II. U.S. isolationism encouraged the British in their policy off appeasement and contributed to French paralysis in the face of the growing threat posed by Nazi Germany.
Isolationism contributed to causing ww2 because it gave Japan the freedom to conquer territory through Asia without interference from the United States.
