Final answer:
Russia left the war due to a revolution that led to political changes and a cease-fire declaration
Step-by-step explanation:
The reason Russia left the war was due to a revolution within the country that caused political changes. This revolution led to the overthrow of the tsarist government and the establishment of a new Soviet government. As a result, Russia declared a cease-fire and focused on internal issues.
Learn more about Causes and consequences of Russia leaving World War I