There are a number of important changes that happened in the Soviet Union during this time. Let us examine a few of them.
The death of Lenin and rise of Stalin. This was hugely important. First, it put into power the man who would dominate the Soviet Union until his death in 1953. This also led to a change from Lenin’s New Economic Policy, which was more tolerant of private property and enterprise to Stalin’s policy of forced collectivization of the entire economy. This change led to millions of deaths as a result of famines that were caused by the drop in farm production due to collectivization.
World War II. World War II was as traumatic for the Soviet Union as it was for any other country. It is true that the Soviets did not lose like Japan and Germany did, but they did have their country invaded and they did lose millions of lives both civilian and military.
Start of the Cold War. After WWII ended, the Soviet Union and its wartime allies fell out with one another. This was a tremendous change for the Soviet Union because it committed the country to a policy of resisting the West and trying to spread communism. These pressures would eventually cause the death of the Soviet system.