What role did russia play during world war i?

Russia entered the war as an ally of Serbia, after the declaration of war on that nation by the Austro-Hungarian Empire and Germany. Russia occupied primarily German forces on the Eastern Front until the Russian Revolution overthrew the Tsar and the new revolutionary government signed a ruinous peace treaty and ended hostilities, freeing a large number of troops for the Western Front. 

However, that was not all. Russian-inspired revolutionary communist ideas spread throughout a more and more disaffected Germany and German military, and was partially responsible for the military mutinies that forced the Central Powers into an armistice.