But what if Tokyo or Seoul were in fact in earnest, and ready to go the final mile to obtain its own arsenal, even at the cost of breaking the alliance with Washington? Would the benefits of pursuing Santoro’s recommended course of action outweigh the costs?

On the cost side, we can assume that wholly opposing either ally’s resolute decision to pursue nuclear weapons would put substantial pressure on our bilateral relations. It might well undermine them entirely if Tokyo or Seoul were truly determined to obtain its own nuclear arsenal, a prospect Santoro is ready to embrace in the event the choice needed to be made. But we must bear in mind the costs of such a course of action, which would likely have enormous strategic implications for the United States. Though U.S. policymakers and experts often like to state that U.S. alliances restrain proliferation among our allies, these alliances are not primarily nonproliferation instruments. Rather, they are geopolitical instruments designed to strengthen America’s position in the world and thus increase American security. Nonproliferation objectives are a part of that broader purpose, but only a part—and should be subordinated to and serve that broader goal.

And, in point of fact, U.S. alliances with Tokyo and Seoul do have far larger and, indeed, more important goals than simply restraining Japan and the ROK’s nuclear ambitions. Rather, U.S. alliances with Japan and South Korea are cornerstones of the whole U.S. position in Asia. The two countries host the bulk of U.S. bases in the region and their defense forces are major multipliers and legitimizers for Washington. Diplomatically, the alliances help anchor the U.S. presence in Asia. Now, if there were no serious challenges to U.S. interests in the Asia-Pacific, abandoning such equities might make sense, since there would be no need for them in such a benignly pacific region. But that is not the case. The United States faces in Asia not only an aggressive, nuclear-armed North Korea but, far more importantly, the rise of a mighty China which Washington will need to balance for the foreseeable future. In order to meet these challenges successfully, the United States will need allies like Japan and South Korea (just as they will need the United States). Terminating our alliances with Tokyo or Seoul would dramatically undermine these broader strategic objectives. Indeed, we could well expect the costs of cutting our alliances in such circumstances to be particularly high since it would likely be precisely intensified North Korean belligerence and/or growing Chinese assertiveness—and the calculation that the U.S. extended deterrent was insufficient to check them—that would impel Tokyo or Seoul’s decision to pursue a nuclear weapons program. Abandoning our alliance just as our ally was demonstrating its increasing skepticism about the sufficiency of our guarantees would likely exacerbate rather than alleviate the very tensions and instability we would be seeking to control.

Thus the costs of unyieldingly opposing such proliferation might be very high. But at the same time the benefits of such an approach might well be relatively low. The primary reason might simply be that U.S. opposition might not translate into much real influence over either Tokyo or Seoul should they elect to “go nuclear.” When the United States squeezed the ROK to halt its autonomous nuclear weapons program in the 1970s, Washington enjoyed overwhelming strategic, military, and economic leverage over Seoul. In the future that will no longer be the case, as South Korea is now one of the globe’s largest and most advanced economies, dwarfing its atavistic northern twin. Washington enjoys probably even less direct leverage over Tokyo, which, despite the Fukushima disaster, boasts one of the world’s largest and most sophisticated nuclear energy infrastructures, a top-notch space launch program, and presides over the globe’s third-largest economy. If Tokyo or Seoul were truly resolved to develop a nuclear weapons capability, especially in the face of threats that drew a decent degree of sympathy or at least quiet understanding in the international arena, it is questionable whether Washington could stop such an effort. Indeed, given that Washington agrees with Seoul and Tokyo about the nature and gravity of the threats most likely to push either one to pursue nuclear weapons of its own, how persuasive would Washington’s opposition be in such circumstances—especially given that the near-tautological premise of such a decision would be that the U.S. extended deterrent was not proving sufficiently credible or effective? The answer is that Washington’s degree of influence might be limited, and perhaps quite limited. Thus, in such instances, even if Washington did not like an autonomous nuclear capability in the hands of Tokyo or Seoul, it might be better off by making a virtue of necessity and ultimately accept it.

Of course we must recall there would be benefits to opposing even resolute pursuit of nuclear weapons by Japan or South Korea and serious costs in not doing so. Washington would indubitably find itself under intense pressure to hold the line against Japanese or South Korean proliferation in order to protect the Nuclear Nonproliferation Treaty-grounded nonproliferation order, which has managed (mostly) to stem proliferation in the decades since its creation. By opposing Japanese or South Korean pursuit of a weapons capability, the United States would demonstrate its commitment to the NPT and to nonproliferation in general, a stance that would undoubtedly resonate beneficially throughout Asia, the Middle East, and elsewhere. But even these real advantages might well not outweigh the costs of an unyielding opposition to Tokyo or Seoul’s pursuit of an autonomous nuclear arsenal.

Needless to say, these are only a few of the most salient factors Washington would need to consider in determining how strenuously to push its nonproliferation policies in the event of a Japanese or South Korean weapons program. Washington would also have to take into account the economic realm, for example. The essential point, however, is simply that Washington’s cost-benefit calculus might well, in certain circumstances, tell in favor of relenting to Tokyo or Seoul obtaining an independent nuclear weapons arsenal.

This is emphatically not to say that the United States should always or presumptively support a Japanese or South Korean nuclear weapons program. To the contrary: in many eventualities, an attempt by either nation to pursue nuclear weapons would be decidedly against U.S. interests and thus should result in intense U.S. opposition. This would clearly be the case if Japan or South Korea fell under an aggressive and hostile government and even in less dramatic cases, such as if the security threat from North Korea and China receded. In such cases pursuit of nuclear arms by Tokyo or Seoul would justifiably provoke Washington to prompt a serious showdown in alliance relations—and might well be grounds for termination of the alliance if Tokyo or Seoul refused to cooperate. And in almost all cases it would be proper and sensible for Washington to oppose autonomous Japanese or South Korean programs, even if not to the hilt. At the very least, allies like Japan and South Korea should—and certainly do—understand that pursuit of independent nuclear weapons capabilities stand a very good chance of jeopardizing their ties with Washington. This doubt should—and does—act as a brake on any such ambitions.

But the fact remains that there are instances, including quite cognizable instances, in which it would make sense for Washington ultimately to accede to an autonomous Japanese or South Korean nuclear arsenal. This is simply another way of saying that, in determining how to respond to a Japanese or South Korean nuclear weapons program, Washington would need to weigh costs and benefits, examining why Tokyo or Seoul was pursuing such a capability, the broader strategic context the United States faced, how much Washington needed from Japan or the ROK in the unfolding strategic landscape, the likely response of Tokyo or Seoul as well as others actors in Asia and beyond, the degree of U.S. leverage, and so forth. In briefer terms, it is to say that Washington would need to make a geopolitical decision about how to respond.

Of course a geopolitical calculus might well lead Washington to prioritize nonproliferation. But the point of this article is that it might not. David Santoro has commendably made the case for the nonproliferation uber alles point of view. And he has taken that viewpoint to its logical conclusion, which is that nonproliferation virtue should trump geopolitical camaraderie even when strategic logic would guide us to subordinate the former to the latter. Santoro is not wrong that nonproliferation is a great good, much to be guarded and advanced. But, as Raymond Aron put it, the most difficult questions of statecraft are not between “the good and the evil, but between the preferable and the detestable.” Clearly, further proliferation—even to our allies—would represent a blow to our interests. But to determine whether it would be detestable or only lamentable, we would need to examine the totality of the situation, weigh the benefits, costs, opportunities, and risks, and then decide whether prioritizing nonproliferation above the continuation of our alliances made sense.