World War I: The American entry into World War I begins when diplomatic relations with Germany are severed due to its unrestricted submarine warfare.

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. Theatres of war included Europe, the Middle East, Africa, and the Asia-Pacific. One of the deadliest conflicts in history, World War I resulted in an estimated 15 to 22 million deaths, including those in war crimes and genocides. The war also helped spread the Spanish flu pandemic. The conflict saw important developments in weaponry, including the first large-scale use of machine guns, artillery, aircraft, chemical weapons, and tanks.