On April 6, 1917, the United States Congress declared war upon the German Empire; on April 2, President Woodrow Wilson had asked a special joint session of Congress for this declaration.
1917 United States declaration of war on Germany Wikipedia
WHEREAS, The Imperial German Government has committed repeated acts of war against the people of the United States of America; therefore, be it resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the state of war between the United States and the Imperial German Government, which has thus been thrust upon the United States, is hereby formally declared; and that the President be, and he is hereby, authorized and directed to employ the entire naval and military forces of the United States and the resources of the Government to carry on war against the Imperial German Government; and to bring the conflict to a successful termination all the resources of the country are hereby pledged by the Congress of the United States.
In the Senate, the resolution passed 82 to 6. The negative votes were cast by Asle J. Gronna, Robert M. La Follette Sr., Harry Lane, George W. Norris, William J. Stone, and James K. Vardaman. Eight senators did not vote: John H. Bankhead, Nathan Goff, Thomas P. Gore, Henry F. Hollis, Francis G. Newlands, John Walter Smith, Charles S. Thomas, and Benjamin R. Tillman.