The primary events which led to the outbreak of World War II in Europe were German aggressive expansionary policies in Eastern Europe, primarily the invasion of Poland, which triggered the outbreak. A secondary cause was the reluctance of other European powers to stop German aggression before it erupted into a full blown war.
German Chancellor Adolf Hitler frequently railed against the perceived unfairness of the Treaty of Versailles of 1919, and withdrew Germany from the League of Nations in protest. He later sent German troops into the Rhineland which was occupied by France. Although the French had every right to stop him at that point, they failed to do so. Later, Hitler forced a reunion (anchluss) with Austria claiming he had been invited by the Austrian government to do so. When Hitler demanded the Sudetenland, the German-speaking portion of Czechoslovakia, British Prime Minister Neville Chamberlain flew to Munich and met with Hitler. It was agreed that Hitler to retake the Sudetenland for Germany in exchange for Hitler's promise that he would make no more territorial claims. The Czechs were not allowed to participate in the meeting.
The vacillations of Chamberlain and others convinced Hitler that France and Germany were weak and indecisive; and he next made demands on Poland, ostensibly to protect the interests of German speaking people living in the Polish corridor. At this point, Britain and France more decisively warned him not to proceed against Poland. Rather than heed the warnings, Hitler signed a non-aggression pact with the Soviet Union which contained a secret proviso for the division of Poland between the two powers. Hitler at this point, ready to proceed against Poland and unconcerned about a war, told his generals
My only fear is that at the last moment, some dirty dog will come up with a mediation plan
The German invasion of Poland, Operation Yellow, commenced on September 1, 1939. Two days later, France and Britain issued ultimatums to the Germans to withdraw immediately from Poland. The ultimatums were ignored, and war was declared.