What part did Stalin play in the Cold War?

Stalin was one of the key figures in the early Cold War. Even before the end of World War II, it was apparent that Stalin had designs on securing Eastern Europe as a buffer zone against the West. He authorized the murder of Polish officers in the Katyn Forest in order to try to stifle Polish leadership after the war. Even as Stalin's armies were liberating Eastern Europe from the Nazis, his armies served as the new occupying forces, and they installed puppet governments that would be loyal to the Kremlin. Stalin also authorized the espionage needed to secure nuclear secrets from the United States.
By secretly detonating a nuclear bomb, he helped to create a nuclear arms race between the United States and the Soviet Union.  Stalin also captured German nuclear scientists and used them to develop the Soviet nuclear program after the war.  Stalin insisted that Eastern Germany be stripped bare of machinery in order to rebuild the Soviet Union—this would create a development gap between East and West Germany and would eventually necessitate the Berlin Wall, one of they key places of the Cold War.
Stalin displayed his military might in order to appear powerful, though to the West the Soviet Union appeared belligerent and allowed American politicians to insist on military preparedness after World War II. Stalin's crackdown on dissenters and people of faith also fueled Western animosity.