Image courtesy German Federal Archive
World War II had a number of different causes - ideological, political and economic.
Extreme nationalism helped to unite the populations in Germany, Italy, and Japan against foreigners and some groups within the countries.
The Treaty of Versailles was a failed agreement because it severely punished Germany for World War I which backfired. Instead of only weakening Germany's military, it created dire conditions in the country which allowed for extremist political parties and ideologies to take root. As a result, Hitler and the Nazi Party took power in the early 1930s.
The League of Nations, another product of the Treaty of Versailles, also failed in its mission to reduce conflict among member nations. When Japan invaded Manchuria and Italy invaded Ethiopia, the League had no means to back up its demands leaving it powerless to stop the aggressions of these fascist nations. Germany and Italy also withdrew from the League during the 1930s.
The Western powers also did little to curb Germany's increasing aggressions. When Germany occupied and then annexed Austria in 1938, there was little response from other countries. The United States and most of Europe were still in the throes of the worldwide depression and had adopted a policy of isolationism and pacifism. When Hitler looked to annex the Sudetenland, Europe finally looked to intervene.
Hitler met with the British Prime Minister Neville Chamberlain in Munich where Chamberlain appeased Germany by allowing it to take the Sudetenland from Czechoslovakia. In return, Hitler promised that he would not take over any more territory. Within months, Hitler took over most of the rest of Czechoslovakia showing that appeasement failed.
Meanwhile, Hitler and Stalin shocked the world by signing the Nazi-Soviet Non-Aggression Pact which included secret plans to divide up eastern Europe. Hitler ignored warnings from France and Great Britain and invaded Poland in September 1939, which started World War II in Europe.