After the war Germany split into East Germany & West Germany. The USSR dominated in East Germany, the western allied nations dominated in West Germany. But- the capitol city, Berlin, was in East Germany. So, Berlin got split up, too. Certain parts of the city were controlled by either the USSR or the US or Britain, etc. Now, the USSR and the western allied nations did not get along at all, so we couldn't just drive supplies like food and medicine and mail into our sector of Berlin because we weren't allowed through the USSR-dominated areas. So, since we couldn't go through, we went over. The Cold War was a messed up time.
The reason why there was a Berlin Air Lift after the war was due to the delays in shipment of food and industrialized equipment. There was also the need to regulate the access of Berlin by the other policy makers.