In 1916, Woodrow Wilson won re-election as president with a powerful slogan. "He kept us out of war." A massive war had been raging across Europe since 1914. Millions of soldiers had already died. Most Americans wanted nothing to do with it. Wilson agreed. But by early 1917, everything changed. Germany began sinking American ships with submarines. Then a secret message was discovered. Germany had asked Mexico to attack the United States. The message, called the Zimmermann Telegram, shocked the nation. On April 2, 1917, Wilson stood before Congress and asked them to declare war on Germany. He said the world needed to be made "safe for democracy." Congress voted yes. Within months, millions of American soldiers shipped across the Atlantic Ocean. They helped the Allies -- Britain, France, and their partners -- push back the German army. The war ended on November 11, 1918. After the war, Wilson tried to create a group called the League of Nations. He wanted countries to talk out their disagreements instead of fighting. The idea was brilliant, but the U.S. Senate refused to join. Many Americans still wanted to stay out of world affairs.