Why did Russia leave World War I?

Bolsheviks.

Explanation:

A revolutionary group that became a communist party and took over Russia. They signed a treaty and removed themselves from the war of WWI.

Social and economic chaos led to revolution and the fall of czardom in Russia in 1917. In 1918, the Bolshevik government signed a peace treaty with Germany (the Treaty of Brest-Litovsk). It got Russia out of the war, ending the Eastern Front to the dismay of France and Britain. The Bolsheviks wanted to consolidate power because it was seen as a priority over the continuation of a conflict they judged to be an "imperialist" war. The peace of Brest-Litovsk supposed colossal territorial losses for Rusia: they handed over Finland, Poland, Ukraine, the Baltic republics and Transcaucasia to the Central Powers; Russia ceded one third of the tsarist empire's population, one-third of its agricultural land and three-quarters of its industries.