At the beginning of WWI, the major Entente Powers (France, Britain, and Russia) made an agreement that none of them would make peace with the Central Powers (Germany and Austria-Hungary) without the others. During the Napoleonic Wars of the previous century, one problem with the coalitions is that one of the powers of the coalition, after being defeated on the battlefield, might come to a separate peace with Napoleon. This allowed him to dismantle coalitions piecemeal. Hence why the Entente Powers made the arrangement.

During the war, this agreement had the effect of benefiting the Entente in that when one was struggling (France in 1914 or Russia in 1915), the others knew that they still wouldn't leave the war.

However, in March of 1917, Russia's Tsar was overthrown. This revolution resulted in the establishment of something called the Provisional Government. It was a pseudo-democratic government that had a tenuous hold on power and was forced to make allies with such groups as the Bolsheviks. The Provisional Government, which included some who had been near to centers of power in the Tsar's regime, chose to continue the war. This is for a number of reasons, such as a desire to win the lands agreed upon throughout the war (such as the Dardanelles), desire to maintain the support of the other democratic powers, etc.

Unfortunately for the Provisional Government, their bid to remain in the war failed. They launched an assault in 1917 which served only to cause an increase in the number of soldiers leaving the front. With the government having failed to solve one of the primary grievances which led to the March Revolution, there was again a simmering level of resentment. Therefore, in November of 1917, the Bolsheviks took the opportunity to overthrow the Provisional Government and establish their own government. It is a rather interesting affair, but two of the primary results for this topic are that the Bolsheviks were unabashedly supportive of leaving the war and the November Revolution led to the Russian Civil War. This war was fought between the Reds (Bolsheviks) and Whites (anyone who considered themselves to be against the Bolsheviks).

As the Bolsheviks remained in power in early 1918, they began talks with Germany for pulling out of the war. It seems that a goal of theirs was simply to stall until the revolution spread to Germany (something they believed was inevitable). However, during this period, the German's caught on and began swallowing up vast quantities of Russian territory while facing an almost non-existent Russian army. This forced the Bolsheviks to change course and on March 3rd of 1918, signed the Treaty of Brest-Litovsk. This treaty gave the Germans, among other things, the lands in Russia that they had seized.

All of this resulted in three things which would lead to Russian losses after the war. First, Britain and France (with the addition of the US) remained supportive of the forces loyal to the old Provisional Government. As the Russian Civil War continued, they supplied and even sent forces in support of the Whites. Hence, the Western democracies were not at all allied with the Bolsheviks. Second, Britain and France felt betrayed by their ally leaving the war in spite of the original agreement not to make a separate peace. They felt that making such a peace meant that any promises made were nullified. Finally, the Bolsheviks had ceded control over much of western Russia to a foreign power.

When in November of 1918 the armistice was signed and peace talks later began, one of the key points was what to do with the Treaty of Brest-Litovsk. Interestingly, the treaty was accepted by the Allied Powers (Britain, France, Italy, and the US). However, since the beneficiary of the treaty had then been defeated, the Allies felt that they could determine what would happen in those ceded lands. Therefore, a number of states were created using the lands lost by Russia in the Brest-Litovsk agreement, including Ukraine, Poland, and White Russia. Throughout the Russian Civil War, the Bolsheviks were able to take some of these back (Ukraine & White Russia), but not others (Poland, Latvia, Lithuania, & Estonia).

TL;DR

The treaty that the Bolsheviks made with the Germans to leave the war in 1917 was upheld by the Allies in 1918, but the lands were used to create new states out of the former Russian lands.

Sources

Gilbert, Martin. First World War. London: Weidenfeld & Nicolson, 1994.

Keegan, John. The First World War. New York : A. Knopf, 1999.

Marshall, S L A. World War I. New York: American Heritage Press, 1971.