The decision by the Soviet Union to pull out of World War I was a decision that concerned the Allies. The Soviet Union signed the Treaty of Brest-Litovsk in 1918. The fighting against Germany in the Soviet Union would stop. The Soviet Union gave up a significant amount of land. Finland, Ukraine, and Georgia became independent. The Soviet Union gave Germany and Austria-Hungary land in Poland, Latvia, Lithuania, and Estonia.
This treaty also impacted the Allies. Since Russia was one of the Allied Powers, the departure of Russia from fighting in World War I allowed Germany to concentrate its fighting on the Western Front. Germany was able to move troops from the Eastern Front to the Western Front. This gave Germany a better chance of winning the war.
Fortunately, the United States had joined the war and helped the Allies. The Central Powers were eventually defeated, and the Treaty of Brest-Litovsk was nullified by the terms of the Versailles Treaty.