German Foreign Minister Frank-Walter Steinmeier and Secretary of State John Kerry at the Hiroshima Peace Memorial.

Secretary of State John Kerrry visited Hiroshima’s Peace Park on April 10, becoming the highest-ranking American official to visit the city we pulverized with the first use of an atomic weapon to help end World War II. Kerry seemed to indicate President Obama might visit the site, a diplomatically, strategically and emotionally fraught decision for America, Japan and much of the world. Bob Butterworth, a member of the Breaking Defense Board of Contributors, tackles the larger question of whether the United States will take any move to shrink its nuclear force in the foreseeable future. Read on. The Editor.

President Obama may try to bring his 2009 Prague Agenda full circle next month with a visit to Hiroshima, where the first nuclear weapon was used. A full circle (or zero!) can also describe the further progress made toward the Prague policy centerpiece, reducing the number of nuclear weapons since New START was signed in April 2010. Those efforts are likely to remain inconsequential in the face of four countervailing imperatives: strategic deterrence; defensive relevance; allied reassurance; and policy-program coherence.

The strategic deterrence factor is straightforward. Since 1961, the US has maintained one fundamental imperative: ensuring no opponents can preemptively destroy all of its long-range nuclear weapons. In the event of war, those weapons might be used to retaliate or to pre-empt, to attack weapons or command centers, to destroy conventional forces or to retard economic recovery, to be launched en masse or a few at a time.

The number of weapons the US needs would depend on the policy, posture, and targeting doctrine under consideration, as well as assumptions about the effectiveness of the enemy attack. Those numbers can and do differ radically. At one time the Strategic Air Command thought 50,000 were needed; in the 1990s a future undersecretary of Defense for policy concluded that 1,000 would do the job; two years ago an NSC staffer suggested that 20 would be enough for him. Those differences are considerable, but in each case there is a minimum number of weapons that must be maintained in the active inventory.

The US has also long sought to make sure it had appropriate and effective countermeasures to meet any likely challenge. The threat of “massive retaliation” for any provocation never seemed very credible, and as Soviet nuclear forces increased in number and size, it seemed increasingly dangerous. The US worked continually through the Cold War to make sure it had the weaponry and command and control systems to provide more options in response to Soviet provocations than simply “suicide or surrender.” Today, however, many of those inherited options appear excessive for responding to likely regional nuclear threats.

At present there is only one program to make the inherited nuclear arsenal somewhat more apposite: the B-61 mod 12, plans for which anticipate lower maximum yields and reduced nuclear effects while maintaining target lethality through better accuracy. Further proliferation of nuclear weapons (or the desire to deter it) will likely require more variety in the American stockpile, including reductions in yield, tailoring of effects, approaches to safety, and highly precise and survivable delivery systems. The active inventory would then grow larger, expanding to include weapons that are smaller, safer, and engineered for protracted storage.

Similar considerations affect the third imperative — reassuring NATO, Japan, and South Korea that the US does include them in its own nuclear deterrence perimeter. American nuclear weapons (B-61 gravity bombs) are physically present only in NATO. But the US strategic arsenal is psychologically powerful in all three alliances, as shown by the unending arguments over when, whether, and why US nuclear weapons should be withdrawn or forward deployed, about the credibility of the US threat, about the circumstances under which the weapons might be used, about the role of national authorities, about the importance of consultations, and about scores of associated topics.

All allies want to be involved in potentially fateful decisions; all want to make sure that nuclear war will not begin before all other options are exhausted; all want to make sure that a nuclear response is not delayed so long that important damage limitation options are forgone; all want to be certain that credible signaling will inhibit further provocations without triggering overreactions; each wants the unilateral authority to trigger nuclear strikes and to prevent other allies from doing so. These debates can be passionate, but they only matter so long as the US can neither be disarmed nor outgunned nor self-deterred. Meeting that condition requires the US maintain a secure, credible, reliable, effective nuclear force that is appropriate to the challenge.

The preceding three imperatives each embody the fourth—a coherent link between ends and means, a clear view of how recommended policies and programs can enhance national security. That link does not dictate specific details; there is more than ample room for argument over how and when and what should come first in shaping the policies and programs responding to the first three imperatives. Basic to each, however, is that clear link to US national security, a coherence between the ends of national security policy and the means for achieving them. That coherence is evident in parts of the Prague speech that endorsed ongoing US programs for non- and counter-proliferation and for controls over “loose nukes” and other nuclear materials. American security is advanced by restricting the number of other actors who have nuclear weapons and by measures to stop terrorists, thugs, and proliferators from gaining control of nuclear materials.

That link is missing, however, in the parts of the “Prague agenda” that advocate a world without nuclear weapons, adherence to a Comprehensive Test Ban Treaty, and a ban on fissile materials. To what ends would these steps lead? The president’s speech provides no explanation of how they would improve US national security or make the world in general safer. The topic may well deserve research and study, but the conclusion is far from obvious. In these matters, too, the Prague agenda will remain ineffective.