What allied power pulled out of the war in 1918 after signing a separate peace agreement with Germany?

In 1918, Russia, one of the Allied Powers, withdrew from World War I after signing the Treaty of Brest-Litovsk with Germany. This separate peace agreement was signed in March 1918 and came about following the Bolshevik Revolution, which led to significant political changes in Russia. The treaty resulted in substantial territorial losses for Russia, allowing Germany to concentrate its forces on the Western Front.