America's monopoly on nuclear weapons determined their subsequent global position and strategy—in effect, it made U.S. hegemony possible, if not inevitable, without commitment of U.S. troops.  This also put much of the war-making/strategic power in the hands of the President, and so strengthened his position at the expense of Congress.

Four great facts dominated international politics in the late 40s:  the atomic bomb; rise of communism, led by Stalin; post-war strength of U.S. vis-à-vis other nations; weakness of the other powers.

Was conflict between the Soviet Union and the U.S. inevitable?  Both had expansionist ideologies.  There is a question of whether Soviet expansion was a strategy of conquest or one of defense.  George F. Kennan , assistant to Ambassador Harriman in Moscow, wrote in Feb. 1946 of the apparent Soviet strategy of aggrandizement; he later published this private message under the signature "X" in the journal, Foreign Affairs, calling for a policy of "containment" with respect to communism, though he was not specific on whether or not he meant that containment should be military or diplomatic—later he would say he meant diplomatic, but the foreign policy "Establishment" used it to mean military.

Kennan's call for containment formed the basis for the Truman Doctrine , which committed the U.S. to policy of both diplomatic and military containment, and led directly to our military involvement in the Greek civil war and later in the Korean War.  The Marshall Plan was a part of this strategy of containment, inasmuch as it strengthened Western Europe against radicalism (communism) and made markets for U.S. goods--economic stability would avert political crisis.

At the end of the war, the U.S. enjoyed tremendous economic and military advantages, and her policy of global containment reflected this peculiar atmosphere of confidence.

The United States committed itself indefinitely to world responsibility during the brief halcyon time when its resources seemed not merely great but superhuman.  Psychologically, this margin of effortless supremacy came to seem the normal state of affairs both to the makers of American policy and to the electorate whose expectations they first set and then would disappoint at their peril.  The first effects of the contrast between expectations and reality had begun to show itself before the 1940s was out.

Go to CNN-Interactive and visit the Iron Curtain  interactive map.