Executive Summary: Berlin Blockade
At the end of WWII, the allied victors divided up Germany to be split amongst them. Berlin was divided amongst the victors as well even though it was within the Soviet Zone. Stalin was focused on gaining strength within his sphere of influence in Easter Europe and within its controlled boundaries. Stalin feared his communist ideals would be compromised by the western nations if they were allowed to control parts of Berlin (because the city was within Soviet controlled Germany).
Pre-play communication played a role with the allies deciding who gets what part of Germany and Berlin. The allies had imperfect information on what Stalin had planned for after the war (the allies figured things would go as planned or order would return to the status quo). Stalin was the aggressor while the Western Allies were the defenders/deters. The blockade by Stalin was meant to keep Western influences out and the US saw it as an opportunity to prove its commitment to democratic nations. The US was determined to spread democracy throughout the world and not allowing this blockade to keep US aid out showed the world the US would do whatever it takes to support its allies. The blockade was a bargaining situation in which Stalin attempted to improve Soviet/communist influence by keeping the US and other democratic nations out.