World War I: The United States declares war on Germany.

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The United States declared war on the German Empire on April 6, 1917. President Woodrow Wilson asked a special joint session of the United States Congress for a declaration of war on April 2, 1917, which passed in the Senate on the same day and then in the House of Representatives four days later on April 6. Wilson signed it into law the same day, making the United States officially involved in the First World War.