What caused world war ll to break out?

The official beginning of WWII was caused by Hitler's invasion of Poland on September 1, 1939. Britain and France issued an ultimatum for Germany to leave—once this ultimatum was ignored, both nations declared war on Germany.
This is the commonly accepted answer, but consider that Japan, Germany's partner in the Tripartite Pact as of 1940, had been at war with China since 1931. Italy, Germany's ally, had been annexing land in Africa since 1936. Hitler annexed Austria and Czechoslovakia before invading Poland. The official beginning of the war is September 1, 1939, but there were aggressive moves all over the world before this time.  
The main causes of the war are extreme nationalism and greed. Germany, Italy, and Japan felt cheated by the Versailles Treaty, Germany most of all since it had to assume the responsibility for the war and pay reparations. All three of these nations put military-minded strongmen in charge who would restore the prestige of the nations. All three wished to annex lands that would prove to be beneficial to their growing populations in terms of places to live and raw materials.