The major combatants in World War I were the Central Powers of Germany, Austria-Hungary, Bulgaria and the Ottoman Empire versus the Allied Forces of Great Britain, France, the United States, Russia, Italy and Japan. In addition, nearly 20 other countries and several colonies joined the conflict.
These alliances developed around a tangle of treaties signed in the late 19th century. While the treaties were intended to stabilize Europe and prevent conflicts, in reality, they drew more countries into a war that probably should have simply been a regional conflict in the Balkans. Because so many of the main combatants were also imperial powers, colonies and protectorates from across the world were drawn into the war, expanding it into the first global conflict in history. Nevertheless, for the largest part of the war all combatants were in a stalemate, with battle lines shifting back and forth over the same few miles of land. Only when the United States entered the war in its last year did the balance of power shift decisively toward the Allies.
World War I was fought primarily on the Atlantic Ocean, where Germany sought to blockade British and other European ports, and in the trenches of Europe. Trench warfare was particularly gruesome; weaponized poison gas and archaic tactics drove solders out into the open, where they could be shot, and the unsanitary conditions led to large numbers of dead from the flu and other diseases.