One cause of world war one was the National self-interest of the western allied countries. The allies were too busy with issues regarding their own countries and they did not have the time or the resources to intervene in issues that were not directly affecting them. The great depression and the war had left the allies economically weak and the population still had strong memories of the horrors of war. A politician taking action and sending their own armed forces to danger, for something that was not directly connected with the allies, would be likely to loose votes in the next election. This made the Allies take a defensive foreign policy and not intervening in disputes that could lead to war. This became a cause of World War II because aggressive actions by nations were not opposed by the allies. The League of Nations could therefore not react forcefully because they were too weak without Britain or France.
An example of National Self-Interest was the Japanese invasion of Manchuria. In 1931 the Japanese staged an attack on one of their railways in eastern China and used this was an excuse to invade and take over Manchuria. The League sent Lord Lytton to investigate the conflict. The "Lytton Report" was published in October 1932, causing Japan to withdraw from the League. The report declared Japan as the aggressors and the League had to react. England and France, the backbone of the League, were too occupied with controlling the Great Depression in their country to raise money to move their navy to solve a dispute that did not really involve them. Without the western Allies, the League could not react forcefully and Japan took Manchuria without major consequences. Japan later used Manchuria as a base to launch a full-scale war against China.