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.