Adolf Hitler started World War II primarily due to his ambition for territorial expansion to reunify German peoples and acquire "Lebensraum" (living space) to make Germany economically self-sufficient and militarily secure. He was motivated by resentment against the harsh Treaty of Versailles, which had ended World War I and imposed severe penalties on Germany. Hitler's aggressive policies involved annexing Austria and Czechoslovakia, and on September 1, 1939, Germany invaded Poland, prompting Britain and France to declare war on Germany, thus starting World War II. Underlying causes included Hitler's belief that expansion through military force was necessary because other countries would not willingly return territory, his buildup of Germany's military power in defiance of the Treaty of Versailles, and his ruthless pursuit of Nazi ideology. This was not a sudden conflict but a result of escalating aggression and failed appeasement over several years in the 1930s.