Allied powers, coalition of countries that opposed the Central Powers during World War I. The Allies’ original members of greatest import were the British Empire, France, and Russia. Later the United States and Italy joined the Allied cause, while Russia withdrew. 

Central Powers, World War I coalition that consisted primarily of the German Empire and Austria-Hungary, the “central” European states that were at war from August 1914 against France and Britain on the Western Front and against Russia on the Eastern Front. Germany, Austria-Hungary, and Italy had been parties to a secret agreement, the Triple Alliance, from 1882 until World War I, but Italy entered the war in opposition to Germany and Austria-Hungary. The Ottoman Empire entered the war on the side of the Central Powers on October 29, 1914, as did Bulgaria on October 14, 1915.