Russia's involvement in World War I ends with the signing of the Treaty of Brest-Litovsk.
The Bolsheviks in Russia had hoped that a treaty could be signed on relatively favorable terms for them, thinking that Germany would want to end hostilities in the east in order to focus on the coming American involvement in the west. Instead, the Germans demanded huge concessions which the Russians at first reject, but are eventually forced to accept.
The communists renounce territorial claims to Finland, Poland, Belarus, Ukraine, and the future Baltic States. Germany technically assumes authority over these territories but after Germany's defeat most of these areas become independent. The terms of the treaty are so harsh and so extreme, that the western allies are actually encouraged to fight harder against the Germans.
The Treaty of Brest-Litovsk is taken as a sign of what will happen to them if Germany wins. In Russia the massive land concessions make things difficult for the Bolsheviks as Russian nationalists begin siding against them in a civil war. After Germany surrenders one of the conditions is that they abandon the Treaty of Brest-Litovsk.