Thomas G. Mahnken is a Visiting Scholar at the School of Advanced International Studies, and Duncan Brown is a National Security Fellow at the Applied Physics Lab, both at The Johns Hopkins University.
As many of the posters in the Interpreter debate have noted, extended nuclear deterrence (END) is under increasing strain. To a large degree, this has been driven by a greatly changed nuclear landscape.
First, nuclear weapons have spread. Whereas for decades nuclear weapons were the exclusive property of a handful of powerful, advanced states, today the ranks of nuclear powers include the backward (North Korea) and the unstable (Pakistan).
Today more than ever, it is the weak rather than the strong that seek nuclear arms.
Second, the relationship between nuclear and conventional weapons has also changed, both for the US and for others, including potential adversaries.
During the Cold War, the US looked to nuclear weapons to offset the size and strength of the Red Army. Specifically, we relied upon nuclear weapons to deter a Soviet invasion of Western Europe, and conventional attacks on other allies.
These days, it is the US that possesses conventional superiority over the full range of adversaries. US conventional superiority provides not only a powerful deterrent, but also a motivation for others to acquire nuclear weapons.
Indeed, it is the potency of Washington's conventional arsenal, rather than its nuclear stockpile, that provides the greater motivation for states that are hostile to the US, to acquire nuclear weapons.
Third, there is an imbalance in political stakes between the US and potential adversaries. During the Cold War, the Soviet Union's nuclear arsenal represented an existential threat to the US and its allies. Today we have limited stakes in many potential conflicts: A nuclear blast in a major US city would inflict horrendous casualties; it would not destroy the US.
By contrast, future adversaries are likely to see a conflict with the US as a threat to their survival. Read More
A war on the Korean Peninsula, would not put at risk the existence of the US, even if Pyongyang were to field an ICBM; it would, however, jeopardize Kim Jong-il's regime and the North Korean state.
Nor would a nuclear Iran pose an existential threat to the US. Tehran's clerics could, however, judge that a war with the US could lead to their ouster. Washington's opponents thus have a strong motivation to escalate future conflicts.
Fourth, technology has changed the nuclear landscape, blurring traditional distinctions between conventional and nuclear arms. Whereas commentators on the left have for years feared that fielding more discriminate nuclear weapons would 'conventionalise' them, in fact conventional arms now approach the effectiveness of nuclear weapons.
Soviet military theorists writing in the late 1970s were among the first to observe that precision-guided munitions (PGMs) were being fielded with an effectiveness nearing that of small tactical-nuclear-weapons.
Conventional PGMs are today capable of destroying a wide range of targets that until recently would have required nuclear weapons. In addition, the deployment of ballistic missile defenses now offers the prospect of defending the US, our forces, and our allies against missile attack.
The changed nuclear environment, in turn, suggests that nuclear weapons could be employed in a broader set of circumstances than in the past. The recently published report of the Johns Hopkins University, Nuclear Futures Project, explores some of these scenarios. They include nuclear use arising out of the implosion of North Korea, a conflict across the Taiwan Strait, a failed Israeli pre-emptive strike against a nuclear Iran, the collapse of Pakistan, or a terrorist attack on the US.
A number of these plausible scenarios could call into question US END guarantees. A conflict with North Korea or China, and prospectively with a nuclear Iran, could escalate to involve nuclear strikes on US bases in allied territory, or even (in the case of China) on sovereign US territory such as Guam.
A nuclear strike on Japan arising out of the collapse of the regime in Pyongyang or a conflict across the Taiwan Strait, would lay bare the dilemma that underpins END: extending nuclear guarantees is meant to lessen the pressure on an ally to acquire nuclear weapons, but in so doing, the US puts at risk its own security on behalf of an ally.
According to the US-Japan Defense Treaty, Washington is committed to the defense of Japan; to not do so would mark the end of the alliance. However, a retaliatory strike would invite further nuclear attacks on Japan and potentially against the United States.
In other cases, it is possible to envision other states seeking END guarantees. Should Iran go nuclear, other states in the Gulf region might seek END guarantees from the US. The Gulf Cooperation Council states, would be increasingly vulnerable to Iranian coercion. Iraq, an Arab state with a Shi'a majority on Iran's borders, would be particularly uncomfortable in Iran's nuclear shadow.
Compounding the challenge of the US maintaining END, is its decreasing means of doing so. The US has traditionally underwritten its END guarantees with forward-based forces. However, Washington has decreased markedly the size of its theater nuclear forces since the end of the Cold War. According to the Pentagon, the number of non-strategic US nuclear warheads declined by approximately 90% from 1991 to 2009.
If END is to endure, then the US and its allies will need to work harder at maintaining and bolstering it. This may include declaratory policy, but will also likely require changes in force posture to increase the credibility of extended deterrence. This debate is a good way to foster such a dialogue.
The Nuclear Reactions column is supported by the Nuclear Security Project of the Nuclear Threat Initiative, as part of a wider partnership between the NSP and the Lowy Institute.
Photo, of a US Air Force B-2 Spirit and two B-117A Nighthawks, by Flickr user mashleymorgan.