Which event is associated with the failures of the League of Nations?

The event that is most closely associated with the failures of the League of Nations is the invasion of Poland. The League of Nations was established after World War I with the goal of maintaining peace and preventing future conflicts through collective security and diplomacy. However, when Germany, under Hitler's leadership, invaded Poland in 1939, the League's lack of enforcement power and inability to respond effectively highlighted its weaknesses. The League's failure to prevent aggression from member and non-member states ultimately led to its irrelevance and the outbreak of World War II.

The other options represent significant historical events but do not directly underscore the League's shortcomings in the same way. The Treaty of Versailles established the League but was more about the peace settlement following World War I than demonstrating the League’s failures. The rise of Adolf Hitler is important in understanding the context of world events leading to aggression, but it doesn't pinpoint a failure of the League itself. The Spanish Civil War, while indicative of rising tensions in Europe, was a conflict that the League was unable to intervene in effectively, further showing its limitations, but this is not as directly tied to the League's most catastrophic failure as the invasion of Poland.