* The United States
* The United Kingdom
* France
* Russia
* Germany
These countries formed two opposing military alliances, the Allies and the Central Powers, and their involvement in the war had a profound impact on the course and outcome of the conflict.
The Allies
* The Allies were led by the United States, the United Kingdom, and France, and also included Italy, Japan, and other smaller nations.
* The United States entered the war in 1917 and its involvement was decisive, as its fresh troops and resources helped to turn the tide of the war in favor of the Allies.
The Central Powers
* The Central Powers were led by Germany and Austria-Hungary, and also included the Ottoman Empire and Bulgaria.
* Germany was the most powerful of the Central Powers and its military prowess was formidable, but it was ultimately defeated by the combined forces of the Allies.