The Berlin Blockade (24 June 1948 – 12 May 1949) was one of the first major international crisis of the Cold War, a sustained state of political and military tension between powers in the Western world led by the United States with NATO among its allies, and powers in the Eastern hemisphere, dominated by the Soviet Union along with the Warsaw Pact. This began after the success of their temporary wartime alliance against Nazi Germany, leaving the USSR and the US as two superpowers with major economic and political differences. Back then the world was considered bipolar because these two nations were the two superpowers in the world and were have serious issues in regards to economics, politic ideologies in general.
During the multinational occupation of Germany after WWII, the city of Berlin was split in four zones; each one was occupied by a different nation. The four nations occupying these zones were the US, France, the UK, and the Soviet Union. The USSR's leader, Joseph Stalin, was not fond of this idea and wanted to push the Western allies away so that he would be able to take full control over the city of Berlin and consequently the rest of Germany. The tensions between the US and USSR rose up and even though there was no act of war, this situation they found themselves in became known as the Cold War.
Stalin's real intentions with the Berlin Blockade, as the History Channel's web page describes were to "limit the ability of France, Great Britain and the United States to travel to their sectors of Berlin, which lay within Russian-occupied East Germany" This would give the USSR edge over the Western allies in the fight for control over Berlin and consequently Germany. Joseph Stalin wanted to force them away so that he could have full control over the city of Berlin and later expand to the rest of Germany and surrounding countries.
It is interesting to point out that the US and the USSR were partners short before the Cold War took place.