League of Nations
The League of Nations was the first attempt to assemble the nations of the world in a single body. In this respect it was the forerunner of the United Nations. It was created at the urging of the United States, tinged with idealism, and criticized by nationalists.
U.S. role
Following the entry of the United States into World War I, President Woodrow Wilson paid surprisingly little attention to military affairs, but dominated diplomacy and provided the funding and food supplies that helped to make Allied victory in 1918 possible. In the late stages of the war he took personal control of negotiations with Germany, especially with the Fourteen Points and the Armistice that amounted to a German surrender. The Fourteen Points was his program to permanently end conflicts that lead to wars, and he went to Paris in 1919 to implement them and especially to create the League of Nations. Wilson had to compromise with French leaders who wanted revenge against Germany, and deal with overlapping and conflicting claims of national self determination, He played the single most influential role in shaping the Treaty of Versailles, with special attention on creating new nations out of defunct empires. He succeeded in getting most of the treaty he wanted, but faced a severe battle regarding Senate ratification. Wilson collapsed with a debilitating stroke in 1919, as the home front saw massive strikes and race riots, and wartime prosperity turn into postwar depression. He refused to compromise with the Republicans who controlled Congress after 1918, effectively destroying any chance for ratification of the Treaty of Versailles. The League of Nations went into operation anyway, but the U.S. never joined.
Wilson's idealistic internationalism, whereby the U.S. enters the world arena to fight for democracy and liberalism, has been a highly controversial position in American foreign policy, serving as a model for "idealists" to emulate or "realists" to reject for the following century.
1920s
Most historians portray 1924-29 as no more than a temporary lull between the two storm periods of imposed "peace" settlements and bitter conflicts in 1919-23 and the Great Depression, destruction of Versailles, and rise of Hitler in 1929-33. However Cohrs (2006) argues that the Dawes Plan, London Conference, and Locarno treaties of 1924-25 constituted instead a new Euro-Atlantic peace settlement and international system and established a new European concert. This system was much more important than the League, Cohrs argues. Under combined British-American aegis, both France and Germany adopted the spirit and politics of productive mutual compromise. What stunted and finally undermined the system was above all the failure of both hegemons, especially the U.S., to actively support and develop it.