On April 6th 1917, the US declares war on Germany via a resolution of the Congress:
Joint Resolution declaring that a state of war exists between the Imperial German government and the government and the people of the United States and making provision to prosecute the same:
Whereas the Imperial German government has committed repeated acts of war against the government and the people of the United States of America, therefore be it resolved by the Senate and the 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 is hereby authorised 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 of the resources of the country are hereby pledged by the Congress of the United States.
Champ Clark (Speaker of the House of Representatives)
Thomas Marshall (Vice-President of the United States)
Woodrow Wilson (President of the United States)