The Entente Powers were the countries at war with the Central Powers during World War I. The members of the Triple Entente were the United Kingdom, France, and the Russian Empire; Italy entered the war on the Entente in 1915. Japan, Belgium, Serbia, Montenegro, Greece, Romania and the Czechoslovak legions were minor members of the Entente.

The United States declared war on Germany 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 Great Britain, 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 From early 1917 the BWC was superseded by the Imperial War Cabinet, which had Dominion representation.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 Generals John Monash and Arthur Currie, respectively, who reported in turn to British generals.^{[citation needed]}

World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.