The major global conflict that the League of Nations failed to prevent was World War II. Despite its establishment after World War I to promote peace and cooperation among nations, the League struggled with enforcement and consensus-building among member countries. Key nations, such as the United States, did not join, weakening its authority. The League's inability to curb aggressive actions by totalitarian regimes in the 1930s, including Japan’s invasion of Manchuria and Italy's aggression in Ethiopia, ultimately showcased its ineffectiveness, leading to the outbreak of World War II in 1939.