Why did the League of Nations fail to prevent later aggression and war?

The League of Nations failed to prevent aggression and war primarily because it lacked the necessary mechanisms and political will to enforce its own principles of collective security.12

Several critical factors contributed to this failure:

• Absence of the United States: The U.S. Senate’s refusal to ratify the Treaty of Versailles meant the world’s greatest economic power never joined the League. This absence undermined the credibility of economic sanctions, which were intended to be the primary deterrent against aggressors.31
• Structural Weakness: The requirement for unanimity in the Assembly and Council often led to compromise or delay rather than decisive action.4
• Lack of Enforcement: While the Covenant provided for military intervention, the League possessed no independent military force and relied on member states to provide one. Major powers, often preoccupied with their own economic crises or pursuing policies of appeasement, frequently failed to act against clear violations of the Covenant, such as the Japanese invasion of Manchuria or the Italian invasion of Ethiopia.1256

Ultimately, the League could not master the crises that led to World War II, as its members prioritized national interests over the collective obligation to maintain international peace.52