Why did the League of Nations fail to prevent World War II?

The failure of the League of Nations to prevent World War II stemmed from fundamental structural weaknesses and a lack of political will among its member states.

Structural and Membership Limitations

The League’s effectiveness was crippled from the start by the absence of the United States, which refused to join, thereby undermining the credibility of collective security as a deterrent. Furthermore, the requirement for unanimity in the Council and Assembly often led to compromise or paralysis rather than decisive action.21

Failure to Address Aggression

The League proved unable to check the expansionist policies of aggressive powers:
* Manchuria: When Japan invaded Manchuria in 1931, the League’s response was slow and ineffective. Japan ultimately withdrew from the organization in 1933 rather than comply with international demands.23
* Ethiopia: The League’s failure to stop Mussolini’s invasion of Ethiopia was a turning point. Although economic sanctions were initially applied, they were undermined by the British and French governments, who sought to appease Italy, effectively destroying the League's authority.24

Ultimately, the League could not overcome the isolationism of major powers and the aggressive rearmament of nations like Germany, which withdrew from the organization in 1933.23