For three years, a terrible war raged across Europe. The United States stayed out of it. President Woodrow Wilson promised to keep America neutral, meaning it would not pick a side. But staying neutral became harder and harder. German submarines sank American ships crossing the Atlantic Ocean. In early 1917, a secret message was intercepted. Germany had asked Mexico to attack the United States. The message shocked the country. Americans felt threatened. On April 6, 1917, Congress voted to enter World War I. Over four million Americans joined the military. Soldiers trained at camps across the country. Then they boarded ships and sailed to France. American troops arrived at a time when both sides were exhausted. The fresh soldiers helped turn the tide. Factories at home produced weapons, food, and supplies for the war effort. Women took over factory jobs that men had left behind. The war ended on November 11, 1918. The cost was enormous. Over 116,000 American soldiers died. Millions more were killed across Europe. After the war, President Wilson pushed for a new group called the League of Nations. He hoped it would prevent another devastating war. The Senate voted against joining, but the idea later grew into the United Nations.