At the Yalta Conference , Roosevelt , Churchill , and Stalin agreed to the establishment of the United Nations , as well as the structure of the United Nations Security Council , which would be charged with ensuring international peace and security . The participants at Yalta also agreed that the United Nations would convene for the first time in San Francisco in April 1945 in the United Nations Conference on International Organization . Roosevelt considered the United Nations to be his most important legacy . He provided continuous backstage political support inside the United States , and with Churchill and Stalin abroad . He made sure that leading Republicans were on board , especially Senators Arthur Vandenberg of Michigan , and Warren Austin of Vermont . The Allies had agreed to the basic structure of the new body at the Dumbarton Oaks Conference in 1944 . The Big Four of the United States , Britain , Soviet Union and China would make the major decisions , with France added later to provide permanent members of the all-powerful Security Council . Each had a veto power , thus avoiding the fatal weakness of the League of Nations , which had theoretically been able order its members to act in defiance of their own parliaments
United Nations Conference on International Organization
United Nations Conference on International Organization
Presidency of Franklin D. Roosevelt, third and fourth terms