In a feat of technical sophistication and strategic insight, Lieber and Press have modeled a U.S. first strike against Russia. … 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.

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.