The Allied Powers in World War I consisted of France, Russia, Great Britain, Japan, Italy and the United States. They fought against a group of European countries known as the Central Powers that were formed by a treaty called the Triple Alliance.
Britain, France, and Russia created the -Triple Entente- which was a treaty intended to unite the three countries against any potential invasion by the Triple Alliance, despite Britain and France previously having different national and economic goals based on colonialism. The Triple Alliance originally consisted of Germany, Austria-Hungary, and Italy. Italy eventually left the Triple Alliance and joined the Allied powers later in the war. The Triple Entente was joined by Japan and unofficially by the United States later in the war.
Japan entered the war on the side of the Allied powers after Germany refused to relinquish certain territories to China's control, and in doing so, honored the Anglo-Japanese Alliance. The Anglo-Japanese Alliance was a treaty made between Britain and Japan. The United States joined the war in 1917 after German submarine crews attacked shipping trade routes, breaking the neutrality between the countries. The United States remained an associated power to the Triple Entente rather than open allies, under the premise of avoiding escalated conflicts with the Triple Alliance.