IT IS SAID THAT GENERALS are always fighting the last war, and this is no less true of politicians and policymakers. As the first war of the new century begins, America's leaders have been reaching back to the two great struggles of the 20th century, against communism and fascism, to understand this one. Some, like Defense Secretary Donald Rumsfeld, think that the appropriate comparison is to the Cold War. Rumsfeld has said that today's war will be waged more as we fought the Communists than the Axis. On October 4, in Cairo, he declared:

For it undoubtedly will prove to be a lot more like a cold war than a hot war. In the cold war it took 50 years, plus or minus. It did not involve major battles. It involved continuous pressure. It involved cooperation by a host of nations. And when it ended, it ended not with a bang, but through internal collapse. . . . It strikes me that might be a more appropriate way to think about what we are up against here, than would be any [other] major conflict.

In contrast, Rumsfeld's commander in chief seems to have uppermost in his mind the struggle against Nazism. In his address to the joint session of Congress on September 20, President Bush declared: "By abandoning every value except the will to power, [the terrorists] follow in the path of fascism, and Nazism, and totalitarianism."

Which should be our guide (if either) -- the fight against communism or against fascism? This is more than an academic question. Nazism and communism were dissimilar regimes, of different historical and philosophic lineages, and exhibiting distinct political profiles and contrasting international conduct. The response of the United States to each threat was also quite different. Thus whether our reference point is to Nazism or communism will have enormous policy implications.

U.S. foreign policy towards communism was mapped out in George F. Kennan's (or Mr. X's) famous article, "The Sources of Soviet Conduct," published in Foreign Affairs in 1947. Kennan argued that to counter Soviet conduct, one must first come to grips with "the political personality" of Soviet power, which he diagnosed as a product of Communist ideology and historical circumstance. The interplay of these two factors, in his view, caused the Soviet Union to be highly flexible and responsive to outside pressures in pursuit of its goals. The Soviet Union was, in a sense, a rational actor, which, in Kennan's memorable words, "can be contained by the adroit and vigilant application of counter-force at a series of constantly shifting geographical and political points, corresponding to the shifts and manoeuvres of Soviet policy."

It was a brilliant piece of political and psychological analysis, which accurately forecast nearly the entire future course of the Cold War. For almost 50 years, we fought a series of wars along the periphery to check Soviet expansion, aided and funded anti-Soviet proxies, and engaged the Soviets in intricate arms-control negotiations and propaganda one-upmanship. Never did we directly engage them on their own soil, nor they on ours. This containment strategy fit the Soviet Union's political personality -- malevolent but rational, ill-intentioned but cautious -- like a glove.

But such a policy, as Kennan implied in his article, would never have succeeded against a power like Nazi Germany. Kennan contrasted the leaders of the Soviet Union with Napoleon and Hitler, who were deaf to anything but their own megalomaniacal dreams of world conquest. Nazism was irrationalist and anti-Enlightenment to its core, while, as an outgrowth of the Enlightenment -- an extreme outgrowth, to be sure -- Soviet communism was amenable to the persuasion of the carrot and the stick.

And indeed, our foreign policy during World War II was very different from what it was during the Cold War. In his war address of December 8, 1941, Franklin Roosevelt flatly declared, "We will not only defend ourselves to the uttermost but will make very certain that this form of treachery shall never endanger us again." And several years later, in a speech before Congress, FDR declared our war aims to be "unconditional surrender," meaning "the end of Nazism and the Nazi party -- and all of its barbaric laws and institutions." In the Soviet case, we applied a policy of calibrated counter-force; in the Nazi case, we sought the total destruction of fascism as a political entity and ideology. It was a war fought not over decades and by proxy, but swiftly and furiously, by our own arms and men, and ultimately with atomic bombs.