Several interconnected factors contributed to the outbreak of World War II:

1. **Treaty of Versailles**: The harsh terms imposed on Germany after World War I, including territorial losses and reparations, created resentment and economic instability, laying the groundwork for future aggression.

2. **Rise of Totalitarian Regimes**: The rise of aggressive totalitarian regimes in Germany (under Hitler), Italy (under Mussolini), and Japan (under militarists) sought to expand their territories and influence, challenging the existing international order.

3. **Expansionist Policies**: Expansionist ambitions of these regimes, seeking to acquire territory and resources, led to aggressive actions such as Germany’s annexation of Austria and the Sudetenland, Italy’s invasion of Ethiopia, and Japan’s expansion into China.

4. **Appeasement and Policy Failures**: The policy of appeasement by Western democracies, attempting to avoid conflict by accommodating aggressor states, failed to deter aggression and emboldened aggressors.

5. **Failure of Collective Security**: The ineffectiveness of the League of Nations in preventing aggression and the failure of collective security mechanisms to address escalating tensions contributed to the breakdown of international order.

6. **Economic Depression**: The global economic depression of the 1930s exacerbated existing tensions, destabilizing economies and societies, which provided fertile ground for extremist ideologies and aggressive nationalism.

These factors converged to create a volatile international environment where diplomatic failures, aggressive expansionism, and unresolved grievances ultimately culminated in the outbreak of World War II in 1939 with the invasion of Poland by Germany, followed by subsequent conflicts involving multiple nations across the globe.