The main causes of World War II were a combination of political, economic, and social factors, primarily stemming from the aftermath of World War I. The Treaty of Versailles imposed heavy reparations on Germany, fostering resentment and economic hardship, which facilitated the rise of Adolf Hitler and the Nazi Party. Expansionist policies by Axis powers, including Germany, Italy, and Japan, further escalated tensions. The failure of appeasement strategies by Western democracies allowed for aggressive territorial conquests. The invasion of Poland by Germany in 1939 ultimately triggered the full-scale outbreak of war, involving numerous nations and leading to widespread devastation.