The Western European and United States policy of appeasement did contribute significantly to the development of World War II. In all of the steps leading to World War II, appeasement was either used or played a major factor in allowing the aggressors in Europe to gain small but important victories. Slowly, step-by-step, these aggressors were able to gain enough power to begin World War II.
During the 1930s, the United States became isolationist for several reasons. First was because of the Great Depression, which consumed most of FDR's effort, so he had no time for foreign affairs. The second reason was that after a Senate investigation of the events which caused American involvement in World War I, it was concluded that the U.S. fought in World War I because of economic entanglements. Therefore, in the 1930s, Americans wanted to sever all links with foreign countries to prevent themselves to be dragged into another war.
The third reason was that as the likelihood of war in Europe increased, the United States favored isolationism more and more. This isolationism was reflected in FDR's policy's regarding foreign affairs. Because of the Great Depression, FDR had no time to worry about other countries and so the policy of appeasement, which was peace at any price, developed.
It can be said that World War II began in 1931, with the Japanese invasion of Manchuria. After the Japanese bombed their own railroad, they blamed it on the Chinese to justify the attack on Manchuria. This invasion violated every international treaty signed up to that time. The League of Nations, because it was weak, could do nothing but respond by sending in a team to investigate the crisis. The United States also didn't do much, because they just responded with the Stimson Doctrine, which...