Major causes of World War II included aggressive expansionism by Germany in Europe and Japan in Asia.
The Treaty of Versailles, which came out of the Paris Peace Conference after World War I, was very punitive towards Germany. Germany was forced to admit responsibility for causing that war. We call that "the war guilt" clause of the treaty. Germany also was forced to pay large reparation payments to the Allies (who opposed Germany in the war). The German economy and national pride were deeply wounded. The Great Depression was worse in Germany than in America. The bad situation in Germany made it possible for a radical leader like Hitler, making all sorts of bold promises, to win over enough people to rise to power. The rise of Hitler and the Nazis meant Germany became eager for war and conquest, hastening World War II in Europe.
Meanwhile, in Asia, Japan was also aggressively pursuing imperial expansion. In 1931, Japan invaded and occupied Manchuria, the northeastern region of China. The invasion followed an explosion that blew up a portion of railroad tracks near the city of Mukden. The railway was owned by the Japanese, who had invested in development in the region. Japan blamed Chinese nationalists for the explosion, but others thought the bombing may have been done by Japanese military personnel to provide Japan with an excuse for invading and occupying Manchurian territory. The Japanese declared the region to be a new country, independent of China. which the Japanese called Manchuko. In reality, the territory was not independent but was controlled by the occupying Japanese army. The League of Nations condemned Japan in 1933 for the events in Manchuria, but that did little to stop Japan. Japan withdrew from the League of Nations at that time, and by 1937 began further invasions into Chinese territory. and in the early 1940s Japan occupied French Indochina (territory in Southeast Asia). This was the way that World War II came about in Asian territory.
