Which event illustrated the League of Nations' inability to respond effectively to aggression?

The Manchurian Crisis is a key event that highlighted the League of Nations' inability to respond effectively to acts of aggression. In 1931, Japan invaded Manchuria, a region in China, claiming that it was necessary for its security and interests. Despite being a member of the League, Japan acted unilaterally without waiting for the League's approval, demonstrating a disregard for collective security principles.

The League's response was noticeably weak. An investigation was launched, and the League's report condemned Japan’s actions, urging withdrawal. However, Japan ignored this call and eventually withdrew from the League altogether in 1933. The inability of the League to enforce its resolutions or to take decisive action against Japan underscored its limitations and failure in maintaining peace and order, effectively illustrating its ineffectiveness in deterring aggressor nations.

While the other events listed also reflect challenges faced by the League, the Manchurian Crisis stands out due to the clear violation of League principles and the lack of effective intervention or repercussions, ultimately signaling the organization's struggles in addressing international conflicts.