At the end of World War II, the Russian military occupied Eastern Europe, and maintained such until the 1980's. This Union was called the USSR, the Union of Soviet Socialist Republics. Although it is unknown why this occupation occurred, these countries are rich in resources, such as access to the Mediterranean Sea.
Also at the end of this War, the United States established an alliance with the Western nations of Europe.
The United States had just used atomic weapons against Japan. It promoted the end of World War II. After World War II, the Russian military acquired such technology from an act of espionage. Julius and Ethel Rosenberg were convicted of transferring this information. They received the death penalty.
What ensued was almost four decades of intense competition between the United States and Russia concerning nuclear weapons prowess. Among the events was the Cuban Missile crisis. This was the establishment by Russia of their weapons in our neighboring country of Cuba.