Allies of World War I The Entente Powers or Allies ( ; ; ; , " Soyuzniki " ; , " Saveznitsi " ; ) were the countries at war with the Central Powers during World War I . The members of the Triple Entente were the French Republic , the British Empire and the Russian Empire ; Italy ended its alliance with the Central Powers and entered the war on the side of the Entente in 1915 . Japan , Belgium , Serbia , Greece , Montenegro , Romania and the Czechoslovak legions were secondary members of the Entente . The United States declared war on Germany in 1917 on the grounds that Germany violated U . S . neutrality by attacking international shipping and because of the Zimmermann Telegram sent to Mexico . The U . S . entered the war as an " associated power , " rather than a formal ally of France and the United Kingdom , in order to avoid " foreign entanglements . " Although the Ottoman Empire and Bulgaria severed relations with the United States , neither declared war . Although the Dominions and Crown Colonies of the British Empire made significant contributions to the Allied war effort , they did not have independent foreign policies during World War I . Operational control of British Empire forces was in the hands of the five - member British War Cabinet ( BWC ) . However , the Dominion governments controlled recruiting , and did remove personnel from front - line duties as they saw fit . From early 1917 the BWC was superseded by the Imperial War Cabinet , which had Dominion representation . The Australian Corps and Canadian Corps were placed for the first time under the command of Australian and Canadian Lieutenant General s John Monash and Arthur Currie , respectively , who reported in turn to British generals . In April 1918 , operational control of all Entente forces on the Western Front passed to the new supreme commander , Ferdinand Foch . History . The original alliance opposed to the Central Powers was the Triple Entente , which was formed by three Great European Powers : The war began with the Austrian attack invasion of Serbia on 28 July 1914 , in response to the assassination of Archduke Franz Ferdinand . The Austrian Empire followed with an attack on Serbian allies Montenegro on 8 August . On the Western Front , the two neutral States of Belgium and Luxembourg were immediately occupied by German troops as part of the German Schlieffen Plan . Of the two Low Countries , Luxembourg chose to capitulate , and was viewed as a collaborationist State by the Entente Powers : Luxembourg never became part of the Allies , and only narrowly avoided Belgian efforts of annexation , at the conclusion of hostilities in 1919 . On 23 August Japan joined the Entente , which then counted seven members . . The entrance of the British Empire brought Nepal into the war . On 23 May 1915 , Italy entered the war on the Entente side and declared war on Austria ; previously , Italy had been a member of the Triple Alliance but had remained neutral since the beginning of the conflict . In 1916 , Montenegro capitulated and left the Entente , and two nations joined , Portugal and Romania . On 6 April 1917 the United States and its American allies entered the war . Liberia , Siam and Greece also became allies . After the October Revolution , Russia left the alliance and ended formal involvement in the war , by the signing of the treaty of Brest Litovsk in November effectively creating a separate peace with the Central Powers . This was followed by Romanian cessation of hostilities , however the Balkan State declared war on Central Powers again on 10 November 1918 . The Russian withdrawal allowed for the final structure of the alliance , which was based on five Great Powers : Leaders . Brazil . " See main Article : Brazil during World War I " Personnel and casualties . These are estimates of the cumulative number of different personnel in uniform 1914 – 1918 , including army , navy and auxiliary forces . At any one time , the various forces were much smaller . Only a fraction of them were frontline combat troops . The numbers do not reflect the length of time each country was involved . ( See also : World War I casualties . ) Summary of Allied declarations of war on Central Powers . The following table shows the timeline of the several declarations of war among the belligerent powers . Entries on a yellow background show severed diplomatic relations only , not actual declarations of war . Unless stated otherwise , declarations of war by and on the United Kingdom include " de facto " declarations by and on other members of the British Empire . Special case : Insurgent nationalities . Four insurgent nationalities , which voluntarily fought with the Allies and seceded from the constituent states of the Central Powers at the end of the war , were allowed to participate as winning nations to the peace treaties : Sources . See List of World War I books