Defining national interest during war is easy: to defeat the enemy. Things get harder after the war. Wilson discovered this; so did Harry Truman. In each case the trouble was with the Allies, who during war agreed on the primacy of victory but after victory had their own national interests to pursue.
The interests of the Soviet Union after World War II entailed control of its East European neighbors. This was not what the United States had been fighting for, and in denying democracy and self-determination, it constituted an affront to American principles. Whether it constituted a challenge to the American national interest was a separate question. Franklin Roosevelt gave signs of thinking it did not, at least not seriously; Harry Truman, upon succeeding Roosevelt, thought it did.
More precisely, what Truman and his Cold War cabinet considered the challenge to the national interest was the aggressive expansionism they perceived to be inherent in Soviet communist ideology, and which was currently manifested in the Soviet occupation of Eastern Europe. Had Truman and his advisers been convinced the Kremlin had no designs west of the Elbe River, they would have slept more soundly and planned less ambitiously for the future of Western Europe; but instead they chose to interpret trouble in Iran, Greece, and Turkey as leading indicators of trouble farther west. In the March 1947 speech that unveiled the Truman Doctrine, the president described the world as divided between "alternative ways of life," with the communist way attempting to subvert and destroy the democratic way. By Truman's reasoning, the fate of America hung on the fate of Greece and the other countries under threat; it followed that the American national interest required defending Greece and those other countries.
Congress, and apparently the American people, endorsed Truman's definition of the national interest. Truman got the $400 million he requested for Greece and Turkey. Then he got the several billions he requested for aid to Europe. He got the Senate to accept the North Atlantic Treaty, committing the United States to the defense of Western Europe. And although he did not ask for a formal declaration of war, he got the Congress to underwrite his decision to defend South Korea against North Korea when war broke out on that divided peninsula in 1950.
Never in American history had the national interest been redefined so radically and swiftly. Scarcely a decade before, Congress had forsworn the use of force even to defend American ships and citizens against direct attack. Now, the United States was pledged to defend half the world and was actively fighting in a small and intrinsically unimportant country half a world away.
Why the change? Two reasons. First, although World War I had suggested that any major European conflict would eventually embroil the United States, Americans required World War II for the lesson to stick. In the twentieth century, American peacetime connections to Europe—business and financial connections primarily, but also cultural and social connections—were so deep and pervasive that Europe's troubles became America's troubles, and Europe's wars became America's wars. The Cold War commitment to Europe—through the Marshall Plan and the North Atlantic Treaty Organization (NATO), especially—represented an effort to keep America out of war by keeping Europe out of war.
The second reason for the radical redefining of the national interest was simply that Americans could. Had the United States been Luxembourg, Americans never would have dreamed of underwriting European reconstruction or defending South Korea. But after 1945 the United States was the richest, most powerful country in the history of the world. Rich people buy insurance policies the poor decline, both because the rich have more to insure and because they can afford the premiums. The ambitious policies undertaken by the Truman administration represented a form of insurance—a way of keeping Americans richer and more powerful than anyone else.