Ummm...your history is a bit off there.
The GERMANS SENT Lenin back to Russia. Deliberately. To foment revolt.
The Russians, essentially, surrendered to Germany in the Treaty of Brest-Litovsk - they gave up the Ukraine, Western Russia, etc. etc. The fighting Stopped. Then the Germans transferred troops to the Western front for the Spring 1918 offensives.
Without US involvement, the Germans would have won the war. Then the Germans would have eventually outbuilt the Britisn and defeated them at sea, and you likely would have had a long term cold (or hot) war for global domination between Germany and the US.
i liked how my WWI book ends:
It was over. The armistice went into effect at eleven A.M. on November 11. Not everyone on the Allied side was pleased. “No no no!” Mangin exclaimed when he learned of the terms. “We must go right into the heart of Germany. The armistice should be signed there. The Germans will not admit that they are beaten. You do not finish wars like this... It is a fatal error and France will pay for it!”
But they didn't feel the need to do so until the USA was definitely entering the war. The October revolution and Treaty of Brest-Litovsk directly followed the return of Lenin and I postulate that this happened because of Germany's well founded worry over the entry of the USA into the war.