Was tension over Berlin after 1945 a cause or a symptom of the Cold War, or both?
I cannot agree with the above answer. The Berlin issue was a symptom of the tension engendered by the Cold War, but not a cause. The tension between East and West, Capitalism and Communism was already in existence.
Immediately after the end of the war, Joseph Stalin told the Soviet People
The war against fascism is ended; the war against capitalism has begun.
Communism by its very nature envisioned world wide domination. It's anthem (and the anthem of the Soviet Union) was the Internationale. It was this determination to dominate Europe, and then the world, which led to East-West tension and the Cold War itself.
Stalin considered the gateway to Europe to be through Germany, and the gateway to Germany was through Berlin. Berlin was situate in East Germany, although a portion of it (West Berlin) was free (or capitalist.) The Berlin Blockade was an attempt to starve out the city, and force it to succumb to Communist rule, as Stalin could not hope to bring all of Germany under Soviet hegemony until he had done so with West Berlin.
In the 1960's, then Soviet Premier Nikita Kruchschev said that the Berlin issue was a 'bone in his throat;" and shortly thereafter ordered the building of the Berlin Wall to prevent residents of East Germany from reaching the West. Berlin--or at least the capitalist island of West Berlin in the center of East Germany--was a source of continual tension; however it was a symptom of tension which already existed. It just happened to be the focal point of that tension.
Tension over Berlin was most definitely both a cause and a symptom of the Cold War.
The tension over Berlin started because of the fact that there was a clear tension between the Soviets and the Western Allies. Had there been no such tension to begin with, there would not have been a West Berlin and an East Berlin but rather one city in four sectors. Therefore, it is clear that the tension was caused by Cold War tensions more broadly.
However, the tensions over Berlin certainly made the Cold War more tense. The blockade of Berlin and the airlift, for example, were two of the defining moments of the Cold War since they served to ratchet up the tension between the two sides. The fact that the Soviets would try to blockade West Berlin most definitely increased tensions.
So, the tensions over Berlin did not cause the Cold War. But they did cause the Cold War to become more intense.