Lenin and Trotsky both assumed that the Bolshevik seizure of power in Russia would be the catalyst for revolutions in Europe.
Germany in particular looked ripe for revolution as it was starting to lose the war.
The Bolsheviks had promised peace but the peace offered by Germany would lead to the loss of large amounts of land.
This would in turn strengthen Imperial Germany and make revolution less likely.
Bukharin and others agreed that the Bolsheviks should continue the war.
Trotsky proposed "neither war or peace".
He believed Russia should retreat and wait for revolution in the West.
Continuing the war would exacerbate the economic and social suffering of the Russian people and would break a key promise made by Lenin.
Lenin took the pragmatic approach and made Russia leave the war.
He was the one that promised peace if he came to power.
Lenin signed the Treaty of Brest-Litovsk.
This was a pragmatic rather than an ideological decision.
Treaty of Brest-Litovsk
Russia lost 2 million square kilometres of land, a sixth of its population, 26% of its railway lines, 74% of its iron ore and coal supplies and a third of its agricultural production.
Finland, Estonia, Lithuania and Latvia all became independent states.
Bessarabia was given to Romania who was a German ally.
Georgia, Belarus and Ukraine had semi-independent government sets up.
On top of the Treaty of Brest-Litovsk, Russia's former allies were angry at Russia for leaving the war and willing to support Russian groups that opposed the Bolsheviks.