World War II was caused by several interrelated factors in the late 1930s including resentment over the Treaty of Versailles, worldwide economic depression, the rise of nationalist and fascist leaders like Hitler and Mussolini, and the policy of appeasement pursued by Western nations. This allowed Germany and Italy to aggressively expand their territory through the late 1930s. In August 1939, Germany and the Soviet Union secretly agreed to divide up Eastern Europe, allowing Germany to invade Poland on September 1st, 1939, marking the start of World War II in Europe.