The United States did not want to enter World War I but wanted to sell weapons to both sides. In 1915 a German submarine sank a ship carrying Americans called the "Lusitania". This angered Americans, and Germany stopped attacking passenger ships. In January 1917 Germany started attacking them again, and sent the Zimmerman Telegram to Mexico about invading the U.S. The United States joined the war against Germany, and it ended a year later. Wilson worked to create an international organization called the League of Nations. The main goal of the League was preventing war. However, the United States did not join because isolationists rejected the peace treaty. At the end of World War I, a flu pandemic killed millions of people in the U.S. and Europe. After the war, the United States was one of the richest and most powerful nations in the world.