What events started WWII in Europe?

After securing the neutrality of the Soviet Union (through the August 1939 German-Soviet Pact of nonaggression), Germany started World War II by invading Poland on September 1, 1939. Britain and France responded by declaring war on Germany on September 3.

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.