Russia's withdrawal from World War I was based on two primary factors. The first concerned the major military reversals it had experienced in the field. The second stemmed from the internal upheaval and political change that culminated in the Bolshevik Revolution of October 1917.
The Russian military experienced negative outcomes from virtually the beginning of the war, according to the BBC. Even though they were able to mobilize faster than their enemies had anticipated, they were still unprepared, being outgunned and outclassed by their German adversaries. Despite a brief rebound in 1916, things managed to get worse for the Russians. Political disorder at home was exacerbated by charges of treason in the royal palace itself. According to BBC History, even the czar's daughter fell under suspicion of spying for the Germans. By 1917, food riots and mutiny within the ranks threatened to unravel the Russian war effort still further.
By 1917, Vladmir Lenin had returned from exile abroad, and was advocating politically for a full-fledged socialist government. Horrific losses in the spring offensive of that year further intensified anti-war feeling, and by October the Bolsheviks had gained power. With Lenin's promise to the masses of “peace, bread and land,” Russia moved toward a peace settlement with Germany that was formalized with the Treaty of Brest-Litovsk the following March.
Though initially a huge blow to her primary allies, Britain and France, Russia's withdrawal from the war was ultimately offset by the entry of the United States, and by 1918 Germany was forced to the peace table.