The United States Senate declined to approve Wilson's Treaty of Versailles, citing concerns that, among other things, American participation in the League of Nations would mean that American troops would be dispatched to Europe to handle European issues. American forces landed in France in the late summer of 1918.
When members of the Senate believe their concerns have not been satisfactorily addressed, treaties have been rejected. The Treaty of Versailles, which formally concluded World War I, was rejected by the Senate in 1919, in part because President Woodrow Wilson had failed to examine senators' concerns to the deal.