What was identified as a major problem for the League of Nations?

The major problem for the League of Nations was its weakness without America. The League, established after World War I to promote peace and cooperation among nations, significantly struggled in its effectiveness due to the absence of the United States, which was one of the most powerful nations at the time. America's decision not to join the League undermined its authority and credibility, as it denied the organization necessary military and economic power. Furthermore, the lack of American participation limited the League's ability to enforce decisions, implement sanctions, and maintain collective security, which were foundational principles of its charter.

In the context of the other options, strong military backing was not a characteristic of the League; it relied primarily on member nations and had no standing army. While the League aimed for united cooperation among its members, achieving this was challenging due to differing national interests and priorities. Finally, while the League sought public support, its overall popularity did not directly translate into effectiveness, especially when faced with significant geopolitical issues that emerged during the interwar period. Thus, the absence of American involvement fundamentally weakened the League's role on the global stage.