Lenin and his buddies did take over the Provisional Government, but they basically only held sway in Leningrad initially. They needed time to consolidate their position, even if a civil war had never happened. Thus they made peace with Germany as at the time the Germans could basically have marched all the way to Vladivostok if need be since all the Russian soldiers were going home. A peace was needed.

Then came all the trouble of the civil war and the events just prior to that, so there was no chance that SU would have been able to rejoin the war ever.