The primary cause behind the conflagration of World War II was a combination of aggressive expansionist policies by Axis powers, particularly Nazi Germany, which invaded Poland in September 1939. The rise of totalitarian regimes in Germany, Italy, and Japan fostered militaristic expansionist ideologies that violated international agreements. The failure of appeasement strategies and the inability of the League of Nations to maintain peace led to escalating tensions across Europe and Asia. The war expanded as countries reacted to Axis aggression, culminating in a global conflict involving numerous nations, fundamentally reshaping international relations and geopolitics in the 20th century.