The League of Nations was unable to intervene in conflicts that preceded World War II; it was an independent organization that lacked its own military power, and it was additionally limited because the United States never decided to join (also, other major world powers—namely Japan, Italy, and Germany—left the League later on). The League of Nations was also set up so that any decisions that were made had to be unanimous. Because of these limitations, it failed to get involved in several conflicts that led up to World War II, including the Spanish Civil War, the Second Sino-Japanese War, and the invasion of Abyssinia by Italy.

Meanwhile, the Treaty of Versailles (which was drafted at the Paris Peace Conference in 1919) failed to adopt an amendment proposed by Japan that would have ended racial inequality within the League of Nations. As a result, Japan left the League and adopted a heavily militaristic attitude, which played a major role in the events of World War II.

The Treaty of Paris encompasses several agreements that were reached at the end of World War I. In particular, the Versailles Treaty resulted in Germany bitterly giving up territory to France, Belgium, and Poland and losing colonies to the Allied powers. When Germany invaded Poland, World War II began.