Though the U.S. Army was ill-prepared, and Wilson himself had campaigned for reelection in 1916 on the slogan, "He kept us out of war," Americans now entered the greatest and most destructive war in history up to that point. World War I was a significant event in the emergence of the United States as a global power and set in motion many of the important political and social trends that would intensify during and after the war.
Dubbed the Great War or the War to End All Wars, World War I began almost three years before the United States became directly involved. The June 1914 assassination by Serbian nationalists of the heir to the throne of the Austro-Hungarian Empire, Archduke Francis Ferdinand, in Sarajevo had provoked the conflict. As the Austrians determined to punish Serbia for the crime, Russia came to the aid of its Serbian ally.
Germany then stepped in to defend its ally Austria-Hungary (an alliance known as the Central Powers), while France honored its treaty of alliance with Russia. Great Britain, angered by the German Army's invasion of neutral Belgium on its way to attack France and fearing the consequences of a powerful German state ruling continental Europe, joined Russia and France (a coalition that became known as the Allied Powers). By 1915, Italy had joined the Allied Powers, Turkey had joined the Central Powers, and the smaller countries of Eastern Europe had