World War One was primarily between the powers of the Triple Entente and Central Powers, but as we mention in our 5 Major Treaties & Alliances in the Build Up to World War One: Video it wasn’t quite so straightforward.
Beyond the main Entente Powers of (France, Russia and Great Britain), Serbia and Japan entered the war in 1914 on the side of those Allies in August. Russia later declared war on the Ottoman Empire in November 1914, after the latter had allied with Germany (thus placing herself in the Central Powers). France and Britain also declared war on the Ottomans that month.
There were a number of late entrants into the war on the Allied side. Italy had not joined the Central Powers at the start of the war as it was committed to do under the Triple Alliance. Instead, she waited to pick sides, and joined the Allies in 1915 – mainly fighting against Austria Hungary. Romania entered the war in August 1916, but after the collapse of Russia was forced to sign an armistice in December 1917.
The biggest late entrant contributor to the eventual Allied victory was the USA, who declared war on Germany April 6 1917. Given Russia had surrendered to Germany under the Treaty of Brest Litovsk (3 March 1918), the Allies now required more manpower to match Germany on the Western Front. The United States mobilised more than 4 million men, with 2 million on active service by the armistice, suffering 110,000 combat deaths.
Below is a graphic of which countries fought on which side: