The full implications of the ensuing nuclear standoff sank in slowly. As the Soviet Union gradually built up its arsenal, American strategic thinkers and policy-makers awakened to some unpleasant discoveries about nuclear arms. The bomb, too, had a distinctly genie-like quality of looking formidable at one instant but useless the next. Even in the days of American nuclear monopoly, between 1945 and the first Soviet explosion of 1949, nuclear weapons had proved a disappointing military instrument. Stalin had simply declared that nuclear weapons were for scaring people with "weak nerves," and acted accordingly. And once the monopoly was broken, no use of nuclear weapons could be planned without facing the prospect of retaliation.

During the 1950s Dwight Eisenhower tried to squeeze what benefit he could out of the United States' lingering numerical nuclear superiority with his "massive retaliation" policy, but its prescription of threatening nuclear annihilation to gain advantage in far-flung local struggles was never quite believable, perhaps even by its practitioners. By the late 1950s a new generation of strategists was awakening to the full dimensions of a central paradox of the nuclear age: Possession of nuclear arsenals did not empower but rather paralyzed their owners. Henry Kissinger remarked, "The more powerful the weapons, the greater the reluctance to use them," and fretted about "how our power can give impetus to our policy rather than paralyze it."

Here at the core of the riddle of American power in the nuclear age was the very image of the pitiful, helpless giant, a figure grown weak through the very excess of his strength. But the source of this weakness, which was very real, had nothing to do with any domestic cowards, not to speak of traitors, or any political event; it lay in the revolutionary consequences for all military power of the invention of nuclear arms, even if--with a hint of defensiveness, perhaps--the United States now called itself a "superpower." (The H-bomb was first called "the super.") Here was a barrier to the application of force that no cultivation of "will" could change or overcome.

But the policy-makers did not accept the verdict of paralysis without a struggle. Within the precincts of high strategy, the "nuclear priesthood" mounted a sustained, complex intellectual insurrection against this distasteful reality of the nuclear age. Even in the face of the undoubted reality that if the arsenals were used, "mutual assured destruction" would result, they looked for room to maneuver. One line of attack was the "counterforce" strategy of targeting the nuclear forces rather than the society of the foe. The hope was to preserve the possibility of some kind of victory, or at least of relative military advantage, from the general ruin of nuclear war. Another line of attack was advocacy of "limited war," championed by Kissinger and others. The strategists reasoned that although "general war" might be unwinnable, limited war, of the kind just then brewing in Vietnam, could be fought and won. Perhaps not all war between nuclear adversaries had been paralyzed. Thus, the impotent omnipotence of the nuclear stalemate became one more paradoxical argument, in addition to those drummed into the public mind by McCarthy and his heirs, in favor of American engagement in counterinsurgency struggles. And this time the United States, unprotected by the prudence of a George Marshall, did go to war.

The results are the ones we know. American military might was no more profitable when used against rebellious local populations in limited wars than it was in general, nuclear wars. This time, the lessons were learned, and for a while they stuck: Peoples, even of small countries, are powerful within their own borders; they have the means to resist foreign occupation successfully; military force will not lead them to change their minds; the issues are therefore essentially political, and in this contest, foreign invaders are fatally disadvantaged from the outset; if they are not willing to stay forever, they lose.

The Decline of Power

By the late 1970s adverse experience sufficient to illuminate the utterly novel historical situation of the United States in the late twentieth century was in hand. Undoubtedly, it had the biggest heap of weapons of any country. Without question, they were the most varied, sophisticated and effective in the world at their job of killing people and blowing things up. The question was what the United States could accomplish with this capacity.

Certainly, if a conventional foe lacking nuclear arms arrayed itself in battle against the United States, it could be handily defeated. That was the mistake that Saddam Hussein made in 1990 when he sent his army out into the Kuwaiti desert, where it was pulverized from the air. But few wars in fact conformed to this conventional pattern any longer. Of far greater importance was what happened to two kinds of war that had historically been the most important -- wars of imperial conquest and general, great-power wars, such as the First and Second World Wars. During the twentieth century the first kind had become hopeless "quagmires," owing to the aroused will of local peoples everywhere who, collectively, had put an end to the age of imperialism. The second were made unfightable and unwinnable by the nuclear revolution. It was these two limitations on the usefulness of military force, one acting at the base of the international system, the other at its apex, that delimited the superiority of the superpower. (The paradox of impotent omnipotence was even more pronounced for the other superpower, the Soviet Union, which actually disappeared.)

Very possibly, the United States, with all its resources, would have been the sort of globe-straddling empire that Joseph McCarthy wanted it to be had it risen to pre-eminence in an earlier age. It was the peculiar trajectory of the United States, born in opposition to empire, to wind up making its own bid for empire only after the age of imperialism was over. Though it's hard to shed a tear, you might say that there was a certain unfairness in America's timing. All the ingredients of past empires were there -- the wealth, the weapons, the power, hard and soft. Only the century was wrong. The United States was not, could not be, and cannot now be a new Rome, much less greater than Rome, because it cannot do what Rome did. It cannot, in a post-imperial age, conquer other countries and lastingly absorb them into a great empire; it cannot, in the nuclear age, not even today, fight and win wars against its chief global rivals, who still, after all, possess nuclear arsenals.

Even tiny, piteous, brutalized, famine-ridden North Korea, more a cult than a country, can deter the United States with its puny putative arsenal. The United States, to be sure, is a great power by any measure, surely the world's greatest, yet that power is hemmed in by obstacles peculiar to our era. The mistake has been not so much to think that the power of the United States is greater than it is as to fail to realize that power itself, whether wielded by the United States or anyone else -- if conceived in terms of military force -- has been in decline. By imagining otherwise, the United States has become the fool of force -- and the fool of history.

In this larger context the repeated constitutional crises of the last half-century assume an altered aspect. The conventional understanding is that an excess of power abroad brings abuses at home. The classic citation is Rome, whose imperial forces, led by Julius Caesar, returning from foreign conquest, crossed the river Rubicon into the homeland and put an end to the republic. (Thus both the proponents of American empire and its detractors can cite Rome.) But that has not been the American story. Rome and would-be Rome are not the same. Empire and the fantasy of empire are not the same.