As part of the Potsdam agreement soon after the end of World War II in Europe, Germany was divided into four zones of occupation for the French, the British, Americans and Soviets. Berlin, even though it was in the eastern zone, was also divided among these four powers.
After ten years, the agreement stated that all foreign forces would pull out and democratic elections would be held. The Soviets never left, and installed a communist government with East Berlin as its capital.
West Berlin remained connected to West Germany by highways and railways, and the Soviets refused to supply the west part of the city. In 1948, in the early days of the Cold War, the Soviets blockaded West Berlin.
If Britain and America gave in and let the city fall into Soviet hands, then that sent a message that western Europe was vulnerable. We adopted a policy of containment and airlifted supplies into West Berlin rather than surrender it, and rather than start World War III to defend it. This episode became a model for containment during the Cold War.
The history of the Cold War is always a history of two sides and two perspectives. I'm not at all fan of the Russian occupation and the subsequent Socialist government that ruled East Germany for decades after the end of the Second World War, but I am certain that blaming the problems on the Soviets is to deny the real complexity of the issue.
The Western and Eastern powers, simply put, did not trust each other, and the Western powers (the U.S., England, and France) took a number of steps, including a currency reform throughout their occupied sections of Germany, that sent a clear signal to the Soviet Union: it's us against you. The Soviets did not simply block access to West Berlin one day because they felt like it would be a fine thing to do.
This side of the history is extremely well documented. I'm citing wikipedia below because it's a quick and easy source (see the section "Moves Toward a West German State"), but I'm confident that print sources with comprehensive coverage of the Cold War will present a full treatment of how the West and the East were both responsible for the enduring divisions and crises faced by the modern Germany.
The issue between the superpowers was over access to West Berlin.
After World War II, the Allied Powers divided Germany between them. Because Berlin was the capital and was symbolically important, they divided Berlin as well. But all of Berlin was within the Russian zone of Germany.
Partly because of this, the Russians wanted to have Berlin all to themselves. However, the Western Allies did not want to look like they were backing down to the USSR so they insisted on keeping West Berlin.
This led to such incidents as the Berlin Airlift. It also eventually led to the building of the Berlin Wall so that people could not escape from East Berlin.