Definition:

The League of Nations was an international organization founded after the Paris Peace Conference in 1919 whose principal mission was to maintain world peace. The United Nations replaced it after the Second World War in 1946 because of its failure to prevent war through collective security and disarmament, demonstrated in its incapability of preventing aggression by the Axis Powers.

Significance:

The League of Nations is significant for peacemaking and peacekeeping from 1918-1936 because it had failed to maintain international peace. The organization lacked its own armed force (had "no teeth") and depended on the Great Powers (USA, Great Britain) to enforce resolutions, which ultimately failed to retain aggression and resulted in the Second World War.