When 1917 ran around, every major power was exhausted and the war was basically still anybody's to take. Russia was the first country to break first and drop out of the war because the Brusilov Offensive made Russia spend more resources and have them run out. They eventually got stuck. Russia running out of supplies which therefore led to the Russian Revolution where the government got overthrown by it's angry citizens and that new government got overthown a few months later and that new government was the USSR.
