The first impulse of states that acquire nuclear weapons is “simple” deterrence, and the simplest form of deterrence is massive retaliation to existential threats. (India and Pakistan, for example, now embrace declaratory policies of simple deterrence.) The second impulse, made possible by the growth of nuclear capabilities, is to avoid “all or nothing” decisions that could effectively result in the complete destruction of national territory. Thus was born the notion of limited nuclear options.

Here’s how the Nixon administration thought about the problem, as codified in NSDM 242, issued on January 17, 1974:

The United States will rely primarily on U.S. and allied conventional forces to deter conventional aggression by both nuclear and non-nuclear powers. Nevertheless, this does not preclude U. S. use of nuclear weapons in response to conventional aggression.

Should conflict occur, the most critical employment objective is to seek early war termination, on terms acceptable to the United States and its allies, at the lowest level of conflict feasible. This objective requires planning a wide range of limited nuclear employment options which could be used in conjunction with supporting political and military measures (including conventional forces) to control escalation.

Plans should be developed for limited employment options which enable the United States to conduct selected nuclear operations, in concert with conventional forces, which protect vital U. S. interests and limit enemy capabilities to continue aggression. In addition, these options should enable the United States to communicate to the enemy a determination to resist aggression, coupled with a desire to exercise restraint.

Thus, options should be developed in which the level, scope, and duration of violence is limited in a manner which can be clearly and credibly communicated to the enemy. The options should (a) hold some vital enemy targets hostage to subsequent destruction by survivable nuclear forces, and (b) permit control over the timing and pace of attack execution, in order to provide the enemy opportunities to reconsider his actions.

Nuclear deterrence theory works best at the conceptual level. As Hans Morganthau once wrote, “The very purpose of threat and counter-threat is to prevent the test of actual performance from taking place.” But here’s the rub: Deterrence concepts must be credible, which means that they must be translated into operation plans. And when concepts become plans, they strain credulity.

No operational plans are more “incredible” than those for limited nuclear war. In Robert Osgood’s best known book, Limited War: The Challenge to American Strategy (1957), he argues that credibility “requires that the means of deterrence be proportionate to the objectives at stake.” Osgood’s view was challenged at the time by those who believed that credible nuclear deterrence required more punch than the enemy could take. As General Curtis LeMay testified before the Senate’s Airpower Hearings in 1956,

A deterrent force is one that is large enough and efficient enough that no matter what the enemy force does, either offensively or defensively, he still will receive a quantity of bombs or explosive force that is more than he is willing to accept.

The dilemma of proportionality vs. leverage has bedeviled nuclear deterrence theory ever since. Reverse mirror imaging hasn’t helped matters: What is perceived as a deterrent by Country X looks a lot like a nuclear war-fighting force in Country Y. The scales of this debate in the United States tipped in the 1970s as Soviet nuclear capabilities grew. (Aspiring Wonks who want a sense of the debate over “prompt hard-target kill capabilities” might check out Paul Nitze’s Deterring our Deterrent in the Winter 1976-77 issue of Foreign Policy.)

The best and the brightest nuclear deterrence theorists – Nitze, Herman Khan, Henry Kissinger, and Albert Wohlstetter, to name a few – failed to crack this nut. Their arguments were unpersuasive because the entire concept of limited nuclear options rested on heroic and questionable assumptions. As Bernard Brodie wrote in his classic book, Strategy and the Missile Age (1959), “It takes only one to start a total war, but it takes two to keep a war limited.” If the stakes in a confrontation between nuclear-armed states were so great as to warrant a crossing of the nuclear threshold, how would they not warrant upping the ante? How would leaders evaluate threat and damage assessments in the chaos of a nuclear battlefield? Would command and control over nuclear forces remain intact in these circumstances? Would national leaders in such discord as to find themselves in a nuclear war somehow manage to find the same page to keep it limited? And how does the quest for securing advantage accord with the very notion of a limited nuclear war?

Given the stakes involved in potential nuclear confrontations, it would be irresponsible for national leaders to refuse to authorize planning for limited nuclear options. But because so many core questions about crossing the nuclear threshold have no good answers, it would also be irresponsible for national leaders to authorize the employment of limited nuclear options.