In early January 1917, Wilson called for “peace without victory,” meaning he wanted the European powers to end the war peaceably, without further military conflict. By late January, however, peace seemed impossible after Germany proclaimed the resumption of unrestricted submarine warfare: Germany vowed to sink all ships, belligerent or neutral, in a wide zone around the Allies. In response, Wilson cut diplomatic relations with Germany. In the next few months, five U.S. ships were sunk. In the meantime, British intelligence intercepted the Zimmerman Telegram. Sent from the German foreign secretary to the German ambassador to Mexico, the Zimmerman Telegram suggested that Mexico enter the war against the U.S. in return for a German pledge to aid in the restoration of Mexico’s former territories of Texas, Arizona, and New Mexico. Germany also promised to help Japan if Japan went to war against the U.S. The telegram and the continued aggression towards U.S. ships convinced Wilson to break U.S. neutrality and call for war.

In April 1917, the U.S. declared war on Germany. Wilson, who had been elected six months earlier on the slogan, “He kept us out of war,” now charged into the war, proclaiming it necessary “to make the world safe for democracy.”

At the time the U.S. passed the declaration of war against Germany, the U.S. Army included 120,000 enlisted men and 80,000 National Guardsmen. In 1917, Congress passed the Selective Service Act, which required all men from age 21 to 30 to register for military duty. By November 1918, some three million men had been drafted. About 11,000 women served in the Navy, and a few hundred more joined the Marines. Women were invaluable in the noncombat positions open to them during the war. More than 250,000 black Americans served in the war, but racism was strong in the military, and black troops were segregated from white troops, given menial positions, and excluded from the marines altogether.

American involvement in World War I lasted from the summer of 1917 to the armistice that ended the war in November 1918—just over one year. American involvement helped to secure Allied victory: American troops overran heavily fortified German trenches and reinvigorated the British and French war efforts. At the war’s end, the American death toll exceeded 110,000. At home, the outbreak of the Spanish influenza claimed over 500,000 lives in 1918—the worst single U.S. epidemic in history.