I assume that you are referring to the decision made by Gen. Eisenhower during WWII, the one in which he allowed the Soviet Army to take Berlin instead of having the Western Allies do so.
Once the Soviet Union had Berlin and much of Eastern Germany, the basic shape of the Cold War was set. This made it clear that the West and the Soviets were going to have to "live" together in some way rather than simply having their separate spheres that did not interact. This led to such things as the Berlin Airlift, the Berlin Crisis of the late '50s, and the creation of the Berlin Wall. All of these increased tensions between the West and the Soviets.
In this way, Eisenhower's decision ensured that there would be tension between the West and the Soviets because they would have to have a great deal of contact with one another.