World War I was the great armed conflict of 1914-18. Until World War II, it was often called the Great War because it was the most destructive and widespread war the world had ever seen.

World War I began as a local conflict over a minor issue. Eventually it engulfed much of Europe and drew in, directly or indirectly, all the major powers of the world. The first declaration of war was made by Austria-Hungary against Serbia on July 28, 1914. Before the armistice was signed on November 11, 1918, 28 nations (counting the British Empire as one nation) were directly engaged in the conflict.

World War IWorld War I saw many innovations in military technology.

On one side were France, Belgium, the British Empire, Russia, and Serbia; and, later, Japan, Italy, the United States, and 16 other countries. They were called the Allied and Associated Powers, or the Allies. The opposing side consisted of Germany, Austria-Hungary, the Ottoman Empire (Turkey), and Bulgaria. They were known as the Central Powers.

After the war, there were numerous boundary changes in Europe and the Middle East. Four empires the Austro-Hungarian, German, Russian, and Ottomancollapsed.

Austria and Hungary were reduced to small separate states and Czechoslovakia was created from Austro-Hungarian territory in Central Europe. The Kingdom of the Serbs, Croats, and Slovenes (which became Yugoslavia in 1929) was established, comprising Austro-Hungarian lands in the Balkans and the kingdoms of Serbia and Montenegro. Poland, which had been partitioned among the Germans, Austrians, and Russians in the 18th century, was reestablished along its historical borders, Finland, Estonia, Latvia, and Lithuania were freed from Russian domination. In the Middle East, Iraq, Syria, Lebanon, Egypt, Hejaz (a territory within modern Saudi Arabia), Transjordan (modern Jordan), and Palestine were carved out of the Ottoman Empire.

France's quick defeat in World War II has been attributed, at least in part, to the devastation it suffered in World War I. The vast system of overseas holdings of Great Britain began to change from an empire to a commonwealth. The war was at least partly responsible for the success of the Russian Revolution and the rise of Communism. The United States, after the war, its first experience of being involved in European affairs, declined to take a role as a world leader and retreated into isolationism, refusing to join the League of Nations. Many people thought of World War I as "the war to end all wars," fought "to make the world safe for democracy." Because of an overly harsh peace treaty, the weakness of the League of Nations, a worldwide economic depression, and the rise of fascism, the war had the opposite effect. It made the second World War almost inevitable.Important dates during World War I