when was it formed?
10 January 1920
League of Nations
 It was founded on 10 January 1920 following the Paris Peace Conference that ended the First World War, and ceased operations on 20 April 1946. 
what is league of nations? It was the first worldwide intergovernmental organisation whose principal mission was to maintain world peace.
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League of Nations [SEP] Introduction
The League of Nations, abbreviated as LON ( , abbreviated as SDN or SdN), was the first worldwide intergovernmental organisation whose principal mission was to maintain world peace. It was founded on 10 January 1920 following the Paris Peace Conference that ended the First World War, and ceased operations on 20 April 1946. The organisation's primary goals, as stated in its Covenant, included preventing wars through collective security and disarmament and settling international disputes through negotiation and arbitration. Other issues in this and related treaties included labour conditions, just treatment of native inhabitants, human and drug trafficking, the arms trade, global health, prisoners of war, and protection of minorities in Europe.