It sorta started at the Munich Conference, when Hitler demanded the Sudetenland, and the democratic powers gave it to him, to try to save Europe from war. Then Hitler took the Rhineland, a demilitarized zone between Germany and France, they did nothing. Then Hitler invaded Poland, and after plenty of delay (known as the Sitzkrieg, a play on words off of the German strategy of Blitzkrieg), they finally declared war on Germany.