The interpretation upon which containment was based was that of George Kennan, who argued that the Soviet Union was indeed expansionist, but that Soviet leaders would shrink in the face of resistance from the West and move to another area of expansion:
the Kremlin has no compunction about retreating in the face of superior forces. And being under the compulsion of no timetable, it does not get panicky under the necessity for such retreat. Its political action is a fluid stream which moves constantly, wherever it is permitted to move, toward a given goal. Its main concern is to make sure that it has filled every nook and cranny available to it in the basin of world power.
Thus the ideal policy was one of containment, where the United States and its western allies would resist, forcibly if necessary, Soviet expansionism, which was taken as a given. Whether this was based on a mistaken view of the Soviet Union is a question laden with ideological baggage. The bulk of modern historians now at least as much on American expansionism as a cause of the war, while many of Truman's opponents at the time argued that it was too soft in that it allowed Stalin and the Soviets to solidify their gains and recognized a legitimate sphere of influence for Soviet power. They pointed to the hard line that the USSR took on Berlin as well as its engineering of a coup in Czechoslovakia and particularly the invasion of South Korea by North Koreans with Soviet backing.