Why did stalin want to create a “buffer zone”?

Are we talking about Cold War and the after affects of WWII? After the Soviet Union made its last stand in Berlin, Stalin was not going to give it up to France, Britain and the U.S.A. He realized that if he gave up Berlin and the territories Lithuania, Estonia, Latvia, the Chezch, Poland, Romania, Bulgaria etc, to get there, the allies would have a direct path to Russia if any conflict were to happen. . So, Stalin treated the invaded countries as satelite states and put many bases and fortifications. 
But inn the long run his buffer was a success and a failure. A success in the that Russia was never invaded. A failure in that after a while the Russian government had a hard time controling all the satelite states, which eventually broke free from the USSR