What were the lessons of the Manchurian Crisis?

In 1931, Imperial Japan invaded Manchuria, on China's eastern seaboard. This came about after the so-called Mukden Incident, when a section of the South Manchurian Railway—in which the Japanese were heavily invested—was blown up by saboteurs. The Japanese accused the Chinese of carrying out the attack, but in actual fact it was a false-flag operation by the Japanese themselves to provide a pretext for invasion.
After the Japanese invasion, the Chinese government appealed to the League of Nations for help. Japan was a member of the League and, as such, was nominally bound to its provisions. A League commission led by Lord Lytton declared that the Japanese must leave Manchuria. The Japanese responded by walking out of the League of Nations for good.
The most important lesson of the Manchurian Crisis was that without firm, decisive action, aggression would go unpunished. Though united in its condemnation of the invasion, the League was unable to agree on a concerted course of action. Britain and France, the League's main military powers, were unwilling to send troops to confront the Japanese. The League of Nations had shown, as they would do in the case of Abyssinia, that they were incapable of either preventing the forced seizure of foreign territory or holding those responsible accountable for their actions.
Both Hitler and Mussolini learned a valuable lesson from the Manchurian Crisis: the League's chronic weakness and inaction meant that it was powerless to stop any country from carrying out an aggressive, expansionist foreign policy. This encouraged both dictators in their imperialist designs, and the subsequent escalation of hostilities led directly to the outbreak of World War II.