In the 1990s, with the Cold War receding, nuclear weapons appeared to be relics. Russian and Chinese leaders apparently thought so. Russia allowed its arsenal to decline precipitously, and China showed little interest in modernizing its nuclear weapons. The small strategic force that China built and deployed in the 1970s and early 1980s is essentially the same one it has today.

But meanwhile, the United States steadily improved its “counterforce” capabilities—those nuclear weapons most effective at targeting an enemy’s nuclear arsenal. Even as it reduced the number of weapons in its nuclear arsenal, the U.S. made its remaining weapons more lethal and accurate. The result today is a global nuclear imbalance unseen in 50 years. And nowhere is U.S. nuclear primacy clearer—or potentially more important—than in the Sino-U.S. relationship.

China has approximately 80 operationally deployed nuclear warheads, but only a few of them—those assigned to single-warhead DF-5 intercontinental ballistic missiles (ICBMs)—can reach the continental United States. (There is no definitive, unclassified count of China’s DF-5 ICBMs, but official U.S. statements have put the number at 18.) China has neither modern nuclear ballistic-missile submarines nor long-range nuclear bombers. Moreover, China’s ICBMs can’t be quickly launched; the warheads are stored separately, and the missiles are kept unfueled. (Unlike the solid fuel used in U.S. missiles, the liquid fuel used to propel Chinese ICBMs is highly corrosive.) Finally, China lacks an advanced early-warning system that would give Beijing reliable notice of an incoming attack.

This small arsenal fulfilled China’s strategic requirements in the 20th century, but it is now obsolete. The current Chinese force was designed for a different era:when China was a poor nation with a limited role on the world stage, and when U.S. and Soviet missiles were too inaccurate to carry out a disarming strike—even against Beijing’s small force. But China’s international presence is expanding, and America’s counterforce capabilities have soared. Moreover, one of the biggest constraints that would deter American leaders from contemplating a disarming strike is fading away. In the past, a U.S. preemptive attack would have generated horrific civilian casualties, but that may soon cease to be the case.

How the United States achieved nuclear dominance after the Soviet Union collapsed is an open secret. The Navy refitted its entire fleet of nuclear-armed submarines with new, highly accurate Trident II missiles and replaced many of the 100-kiloton W76 warheads on these missiles with 455-kiloton W88 warheads. (One kiloton is the explosive energy released by 1,000 tons of TNT.) The result is an unprecedented combination of accuracy and destructive power, essential for an attack on hardened silos. The Navy also recently tested a GPS guidance system that would dramatically boost the accuracy, and thus lethality, of the submarine missile arsenal.

For its part, the Air Force has improved the guidance systems of land-based Minuteman III missiles. Many of these missiles are also being “retipped” with more-powerful warheads—and more-accurate reentry vehicles—taken from recently retired MX (“Peacekeeper”) missiles. The Air Force has also upgraded the avionics on B-2 bombers. These nuclear-mission-capable bombers are already “stealthy,” but the upgrades improve the planes’ ability to penetrate enemy airspace secretly, by flying very low and using the terrain to shield them from radar.

Perhaps as important, the United States is pursuing a slew of nonnuclear weapons that will provide officials options they may find more palatable if they decide to attack an adversary’s nuclear arsenal. These include precision “bunker buster” conventional bombs, high-speed long-range cruise missiles, and conventionally armed ballistic missiles—each of which could be used to destroy enemy missile silos. Furthermore, Washington is undertaking initiatives—including advances in antisatellite warfare and in wide-area remote sensing, designed to find “relocatable” mobile missile launchers—that will make China’s nuclear forces vulnerable. Even a missile-defense system substantially boosts U.S. offensive counterforce capabilities. Critics of this system are right in claiming that it could not shield America from even a modest nuclear attack (e.g., 25 warheads), because it would be easily overwhelmed by decoy warheads and the “penetration aids” that would accompany an adversary’s missiles. But it could enhance offensive nuclear capabilities, by “mopping up” a small number of incoming warheads that survived a U.S. first strike.

From a military perspective, this modernization has paid off: A U.S. nuclear first strike could quickly destroy China’s strategic nuclear arsenal. Whether launched in peacetime or during a crisis, a preemptive strike would likely leave China with no means of nuclear retaliation against American territory. And given the trends in both arsenals, China may live under the shadow of U.S. nuclear primacy for years to come.

Beyond bolstering the ability to conduct a first strike, the improvements to U.S. counterforce weapons also allow war planners to design nuclear options that will make the weapons more “usable” during high-stakes crises. Nuclear planners face many choices when they consider striking a given target. First, they must choose a warhead yield. The American arsenal includes low-yield weapons such as the B-61 bomb, which can detonate with as little explosive force as 0.3 kilotons (one-fiftieth the power of the bomb that destroyed Hiroshima), and high-yield weapons such as the B-83 bomb, which can yield 1,200 kilotons (80 times the strength of the Hiroshima bomb). For a military planner, high-yield weapons are attractive because they’re very likely to destroy the target—even if the weapon misses by some distance. Low-yield warheads, on the other hand, can be more discriminating, if planners want to minimize civilian casualties.

A second key decision for war planners is whether to set the weapon to detonate at ground level or in the air above the target. A groundburst creates enormous overpressure and ground shock, ideal for destroying a hardened target. But groundbursts also create a lot of radioactive fallout. Dirt and other matter is sucked up into the mushroom cloud, mixes with radioactive material, and, after being carried by the wind, falls to earth in the hours after the blast, spreading lethal radiation.

Airbursts create smaller zones of extremely high overpressure, but they also generate very little fallout. If the detonation occurs above a threshold altitude (which depends on the weapon yield), virtually no heavy particles from the ground mix with the radioactive material in the fireball. The radioactive material rises into the high atmosphere and then falls to earth over the course of several weeks in a far less dangerous state and over a very wide area, greatly reducing the harm to civilians.

In the past, a nuclear attack on China’s arsenal would have had horrific humanitarian consequences. The weapons were less accurate, so an effective strike would have required multiple high-yield warheads, detonating on the ground, against each target. The Federation of American Scientists and the Natural Resources Defense Council modeled the consequences of such an attack—similar to the submarine attack described above—and published their findings in 2006. The results were sobering. Although China’s long-range missiles are deployed in a lightly populated region, lethal fallout from an attack would travel hundreds of miles and kill more than 3 million Chinese civilians. American leaders might have contemplated such a strike, but only in the most dire circumstances.

But things are changing radically. Improved accuracy now allows war planners to target hardened sites with low-yield warheads and even airbursts. And the United States is pushing its breakthroughs in accuracy even further. For example, for many years America has used global-positioning systems in conjunction with onboard inertial-guidance systems to improve the accuracy of its conventionally armed (that is, nonnuclear) cruise missiles. Although an adversary may jam the GPS signal near likely targets, the cruise missiles use GPS along their flight route and then—if they lose the signal—use their backup inertial-guidance system for the final few kilometers. This approach has dramatically improved a cruise missile’s accuracy and could be applied to nuclear-armed cruise missiles as well. The United States is deploying jam- resistant GPS receivers on other weapons, experimenting with GPS on its nuclear-armed ballistic missiles, and planning to deploy a new generation of GPS satellites—with higher-powered signals to complicate jamming.

The payoff for equipping cruise missiles (or nuclear bombs) with GPS is clear when one estimates the civilian casualties from a lower-yield, airburst attack. We asked Matthew McKinzie, a scientific consultant to the Natural Resources Defense Council and coauthor of the 2006 study, to rerun the analysis using low-yield detonations compatible with nuclear weapons currently in the U.S. arsenal. Using three warheads per target to increase the odds of destroying every silo, the model predicts fewer than 1,000 Chinese casualties from fallout. In some low-yield scenarios, fewer than 100 Chinese would be killed or injured from fallout. The model is better suited to predicting fallout casualties than to forecasting deaths from the blast and fire, but given the low population in the rural region where the silos are, Chinese fatalities would be fewer than 6,000 in even the most destructive scenario we modeled. And in the future, there may be reliable nonnuclear options for destroying Chinese silos. Freed from the burden of killing millions, a U.S. president staring at the threat of a Chinese nuclear attack on U.S. forces, allies, or territory might be more inclined to choose preemptive action….