The most important cause of World War II was the aggressive expansionist policies of Nazi Germany under the leadership of Adolf Hitler. Hitler's goal was to create a Greater Germany, which would include territories that were lost during World War I, and to gain more living space (Lebensraum) for the German people. Hitler also had a strong anti-Semitic ideology and aimed to exterminate the Jewish population. These expansionist policies led to the invasion of Poland in 1939, which sparked the war. The Treaty of Versailles, which imposed harsh reparations on Germany after World War I, also played a role in creating the conditions for the rise of Hitler and the Nazi party. However, it was ultimately Hitler's aggressive policies and desire for domination that led to the outbreak of World War II.