* The United States: The US entered the war on April 6, 1917, following Germany's resumption of unrestricted submarine warfare, which threatened American ships and citizens.
* Russia: While Russia had been an Allied power since the beginning of the war, the Bolshevik Revolution in 1917 led to the country withdrawing from the war in March 1918. However, the new Soviet government technically remained on the Allied side until the Treaty of Brest-Litovsk forced them out.
While not as impactful as the US or Russia, other countries joined the Allies in 1917 as well:
* Greece: Greece formally declared war on Germany and its allies on June 28, 1917, joining the Allied cause.
* Siam (Thailand): Siam (now Thailand) declared war on Germany on July 22, 1917, mainly due to pressure from its British ally.
These additions significantly impacted the course of World War I, bolstering the Allies' resources and ultimately contributing to their victory.