1917 The Great War. The United States entered World War I in Dec 1917. It changed Europe forever. It demonstrated how the failure to resolve the fallout from one conflict (World War I) can lead to another (World War II). It also showed how a single event (the assassination of Archduke Franz Ferdinand) can set off a series of chain events and — if not stopped — result in the death of millions. Finally, World War I was the first major war with aerial combat, machine guns, tanks, and radio communication. On the morning of December 9, 1917, after Turkish troops move out of the region after only a single day s fighting, officials of the Holy City of Jerusalem offer the keys to the city to encroaching British troops. On April 2, 1917, President Woodrow Wilson went before a joint session of Congress to request a declaration of war against Germany. Wilson cited Germany’s violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war. On April 4, 1917, the U.S. Senate voted in support of the measure to declare war on Germany. The House concurred two days later. The United States later declared war on German ally Austria-Hungary on December 7, 1917.