In recent years special attention has been brought to bear on new and nontraditional dangers posed by weapons of mass destruction—notably, the acquisition of nuclear weapons by ever more countries, and the specter of nuclear terrorism. As William Langewiesche’s article in this issue demonstrates, these are troubling developments, and the right way to deal with them is not at all clear.

But the news media and the nation’s leaders have almost entirely ignored a startling and perilous development in the old sphere of nuclear confrontation, the one involving nuclear-armed major powers: the U.S. threat to the stability of deterrence. The past fifteen years have seen a profound and dangerous shift in the nuclear balance. For most of the Cold War the steadiness of superpower relations rested on the fact that both Moscow and Washington had nuclear retaliatory forces capable of surviving an enemy attack. Both knew that a nuclear war was unwinnable—an attack by one would surely produce a devastating riposte by the other. During this period both sides constantly and meticulously assessed the balance of terror and devoted enormous intellectual energy and sums of money to recalibrating it in response to even the slightest perceived alterations (a fact of which, as an analyst at the RAND Corporation in the late 1980s and early 1990s, I was very much aware). The doctrine known as mutually assured destruction (MAD) entailed a continuous state of readiness, not to mention nervousness, but it bought decades of superpower peace.

Today, however, one country—the United States—appears to be on the verge of establishing true nuclear primacy. Ironically, America’s nuclear dominance may dramatically diminish its security.

Defense analysts have grown increasingly nervous about the convergence of several strategic developments. In “The End of Mutual Assured Destruction?,” a brilliant and sobering study of military analysis that is being prepared for publication in an academic journal, Keir A. Lieber, a scholar at Notre Dame, and Daryl G. Press, a professor at the University of Pennsylvania and a consultant to the Defense Department and to RAND, have trenchantly surveyed the trends that are troubling the experts. The first is the precipitous erosion of Russian nuclear capabilities. Compared with its forces in 1990, Moscow has 55 percent fewer intercontinental ballistic missiles, 39 percent fewer strategic bombers, and 80 percent fewer ballistic-missile submarines, or SSBNs (the component of a nuclear arsenal most likely to survive a first strike). Moscow itself has stated that its nuclear forces will decline by an additional 35 percent in the coming years, but many experts believe the total Russian arsenal could shrink even more, from about 3,800 strategic warheads today to as few as 500 (the United States currently has more than 5,200). More important than this quantitative reduction, though, has been the even steeper qualitative decline. Owing to financial constraints, Russia can’t ensure unbroken monitoring of American ICBM fields, and can’t plug the holes in its missile-warning networks that render it blind to attacks from U.S. submarines in launch areas in the Pacific. Maintenance, supply, and training deficiencies afflict Russia’s nuclear forces generally and its submarines most crucially. A viable Russian deterrent demands that a number of SSBNs be at sea at any given time and that they successfully evade the U.S. attack submarines that stalk them. But in fact most Russian SSBNs must now remain pierside—the Russians weren’t able to conduct any patrols in 2002 and could carry out only two in 2004. This makes the SSBNs highly vulnerable to a U.S. first strike, and it means that the skills Russian SSBN crews need in order to elude U.S. subs have been greatly vitiated (most Russian crews haven’t been on patrol in years). Largely for these reasons former commanders of Russia’s ballistic-missile fleet warned as long ago as 1998 that their supposedly invulnerable submarines would be detected and destroyed in a conflict with the United States.

Equally noteworthy has been the sluggish pace of modernization in the nuclear forces of China, the country likeliest to emerge as the United States’ most formidable “peer competitor” (to employ Pentagon-speak). Whatever Russia’s vulnerabilities, China’s are far more pronounced. Beijing has no operational SSBNs. Moscow’s arsenal contains approximately 800 missiles that could reach the continental United States; Beijing’s contains about eighteen. Most important, the military personnel devoted to China’s nuclear forces have nothing like the training, experience, and institutional history of even Russia’s—let alone America’s, who have been training and preparing for nuclear war for well over half a century.

But whereas Russia’s nuclear capabilities have decayed, and China’s have remained largely static, America’s have become far more lethal. Although the U.S. arsenal has diminished in size, it has also—reflecting the seemingly exponential progress of the U.S. military’s technological revolution—grown both immensely more accurate and immensely more powerful. The Navy, for instance, has refitted its entire SSBN fleet to carry the new, highly accurate Trident II, a missile whose already stunning precision has been nearly continuously upgraded. The Navy has also deployed 400 W88 warheads, which are nearly five times as powerful as the Trident II’s Cold War—era warhead. Minuteman III ICBMs have been retrofitted with much more accurate guidance systems and higher-yield warheads.

These improvements are inconsistent with the aim of simply deterring an adversary’s nuclear attack—a goal that would require merely a “countervalue” strike on the enemy’s cities. They are necessary for a disarming “counterforce” strike, aimed at pre-empting a nuclear attack—and hence winning a nuclear war. Similarly, the avionics upgrades to the B-2 stealth bombermake sense only if that plane is to be directed against the most technologically sophisticated peer competitor, not against a “rogue state” such as Iran or North Korea. Washington’s intentions can’t, of course, be definitely determined, but as a RAND report on the future roles of the U.S. nuclear arsenal concluded in 2003, “What the planned force appears best suited to provide beyond the needs of traditional deterrence is apreemptive counterforce capability against Russia and China. Otherwise, the numbers and the operating procedures simply do not add up” (emphasis in original).

In a feat of technical sophistication and strategic insight, Lieber and Press have modeled a U.S. first strike against Russia. (Although China is Washington’s most probable great-power rival, the authors argue, Russia presents a “hard case” for their contention that America has achieved nuclear ascendancy.) That model, which they presented at the Council on Foreign Relations in October, has been vetted by most of the top civilian defense analysts. To be conservative, it assumes that U.S. nuclear weapons will perform with much less accuracy and reliability than should be expected. Even so, the authors conclude, a U.S. attack today would destroy the entire Russian nuclear arsenal. To grossly oversimplify: the erosion of Russian capabilities, combined with new, overwhelming warhead yields and the “accuracy revolution” in U.S. nuclear forces, has largely obviated the problems of “fratricide” (the prospect that U.S. missiles on the attack would destroy each other, leaving their targets safe) that once helped make a disarming strike impossible to achieve.

Lieber and Press emphasize that their analysis doesn’t prove that a U.S. first strike would succeed, but it highlights a development that is grave if only because it’s one that prudent planners in Russia and China, who conduct similar analyses, are no doubt already surmising: that their countries can no longer be confident of having a viable deterrent. Surely adding to their alarm is the realization that the nuclear imbalance, troubling enough already, will only grow in the coming years. Washington’s withdrawal from the Anti-Ballistic Missile Treaty and its concomitant pursuit of a national missile-defense system will greatly enhance its offensive nuclear capabilities, because although critics of missile defense correctly argue that it could never shield America from a massive full-scale nuclear attack, it could quite plausibly deal with the very few missiles an adversary might have left to deploy after a U.S. first strike. What’s more, the United States is actively pursuing a series of initiatives—including further advances in anti-submarine and anti-satellite warfare; in missile accuracy and potency; and in wide-area remote sensing, aimed at finding “relocatable” targets such as mobile ICBMs—that will render Russia’s and China’s nuclear forces all the more vulnerable.

To be sure, America’s emerging nuclear hegemony could bring benefits, including potential leverage vis-à-vis our superpower counterparts in such areas of competition as the Balkans and Taiwan. It will also force China to divert defense resources from its power-projection efforts in East Asia. (This, however, would be both a blessing and a curse: “We should expect a new, prolonged, and intense nuclear arms race,” Lieber and Press conclude.) But whether or not America has deliberately pursued the ability to win a nuclear conflict, that capability will increase the risk of great-power war. U.S.-Chinese relations are bound to be edgy or worse for the foreseeable future, and although relations between Washington and Moscow are nowhere near their Cold War nadir, actual and potential strains remain formidable. Each country has nuclear-armed missiles that can be delivered against the other within minutes—and in America’s nuclear-war plans the overwhelming number of targets remain inside Russia. Most important, any shift in the nuclear balance itself will engender a volatility that could cause seemingly small conflicts between countries to quickly spiral.

Confronted with the growing nuclear imbalance, Russia and China will be forced to try to redress it; but given America’s advantages, that effort, as Lieber and Press note, could take well over a decade. Until a nuclear stalemate is restored—if it ever is—Moscow and Beijing will surely buy deterrence by spreading out their nuclear forces, decentralizing their command-and-control systems, and implementing “launch on warning” policies. If more than half a century of analyzing nuclear dangers and “crisis stability” has taught us anything, it is that all these steps can cause crises to escalate uncontrollably. They could trigger the unauthorized or accidental use of nuclear weapons; this could lead to inadvertent nuclear war.

American military preponderance now embraces the entire “spectrum of conflict,” as Pentagon planners put it. That is to say, we’re miles ahead of everyone in every type of warfare. But if that preponderance is leading to a world in which Russian and Chinese launch commanders are fingering nuclear hair triggers, the game may not be worth the candle. Without any public scrutiny or debate the United States has emerged as the nuclear hegemon, in possession of a destabilizing first-strike capability. It does not matter whether this has come about by accident or design, or whether America’s motives are worthy or malign; the condition itself is the problem. The ramifications of this state of affairs are of the gravest significance to America’s security—and the world’s. It’s time for scrutiny and debate to begin.