Upon the outbreak of what would become known as World War I, the United States did not yet have the kind of relationship with Great Britain that would develop following World War II, and there were no treaty obligations such as exist today requiring the United States to go to war in defense of allied nations. In 1914, when the war broke out, public and official sentiments in the United States were strongly isolationist, and there was little to no support for intervening in war now raging across Europe.
Among those resistant to getting involved in the war was U.S. President Woodrow Wilson. Two developments, however, changed the course of U.S. history. One involved German submarine operations in the Atlantic that indiscriminately targeted shipping. President Wilson had issued warnings to Germany about interfering with transoceanic shipping, but, on May 7, 1915, a German submarine sank the British oceanliner Lusitania, killing about 1,200 civilians, including 128 Americans.
The other issue that provoked Wilson into acting was the "Zimmermann Telegram," a Western Union telegram from the German Embassy in Washington, D.C., to Germany's ambassador in Mexico. The telegram was a proposal intended for the Mexican Government inviting Mexico to join Germany in an alliance against the United States. The telegram, intercepted by British intelligence, combined with the sinking of the Lusitania, sparked sufficient anger in the U.S. that the decision was made to join the allied war effort.
President Wilson's efforts, therefore, were not intended to restore peace so much as they were to shape the post-war world in a way that would prevent another such conflagation. In a major speech on January 8, 1918, Wilson laid out his "14 Points" for a new world order where the rights of all nations would be respected, freedom of navigation on the seas would be protected, nations would freely trade with one another, occupied territories would be restored to their rightful country, and a League of Nations would be formed to provide a forum for nations to peaceably negotiate the resolution of international conflicts.
Wilson fought hard to have his proposals included in the Treaty of Versaille, which officially terminated hostilities while setting austere conditions on Germany. He was not initially successful in having the League of Nations included, but a final document did establish the new international forum. Ironically, the United States never became a member of the League of Nations because of opposition in Congress.