Dr Stephan Fruehling is a Senior Lecturer in the Strategic and Defence Studies Program, ANU.

Once again, North Korea's missile program has led the US to make major investments into its missile defence capabilities: a Theatre High Altitude Area Defense (THAAD; pictured) battery will be deployed to Guam and fourteen additional Ground Based Interceptors (GBI) will be deployed in Alaska and California at a cost of roughly US$1 billion. This raises the question of the effectiveness of these systems. It is a question that has bedeviled missile defence for many decades, and it is not easy to answer.

Of the US systems intended for the 'exoatmospheric' (ie. in space) intercept of ballistic missiles, the SM-3 interceptor deployed on US and Japanese Navy ships has had the fewest problems, but it is also the system that is most limited in range and speed. THAAD had to overcome a string of failures in the early years of development. After a series of more successful tests, THAAD now enters service more than a decade late. Still, the delayed introduction of THAAD should be a reason for relative confidence in the system: the Bush Administration rushed SM-3 and GBI into operational service while they were still under development, but THAAD had a much more orderly development process.

Undoubtedly, the GBI's record is worst among the deployed systems and is the focus of most missile defence critics. In tests, it has had no more than a 50% success rate, and the newest 10 of the 30 deployed interceptors are known to be defective. On the basis of these figures, it is easy to declare GBI a technical failure. Indeed, there are few excuses for the dismal project management record of the Missile Defense Agency, and credible voices even among missile defence advocates argue that the GBI is flawed and should be replaced with a different design.

But are these relevant criteria to determine the effectiveness of the GBI, let alone missile defence as a whole?

A 50% chance of intercept can still be acceptable if the alternative is to have no defence at all. Currently, the US plans to fire four or five GBIs at any hostile missile, raising the cumulative probability of intercept to close to 97% (assuming no unexpected, systematic failures).

At more than US$80 million per interceptor, the cost of any engagement would outweigh that of the hostile missile by orders of magnitude, leading some critics to deride missile defence a failure on economic, if not on technical, grounds. But by the same logic, using an US$80,000 Javelin missile to kill a Taliban with a $500 Kalashnikov is hardly cost-effective either. In both cases, the assessment should take into account the value of the soldier, or the city, that might otherwise be destroyed.

North Korea could try to 'outbuild' US defences, of course. The cost of US$80 million interceptors must be set in relation to the US defence budget overall. At the moment, North Korea is not known to have any deployed intercontinental ballistic missiles (ICBMs) capable of reaching the US mainland. Should it field more than a handful in the future, the cost of building commensurate numbers of GBI would be considerable. Certainly, in the case of China or Russia, the US could not afford such an 'arms race' at the current cost, should it ever decide to seek the ability to intercept their whole arsenals.

But effectiveness must ultimately be measured by what the system is intended to achieve at the strategic level. If it is to provide a highly reliable and cheap way of intercepting large numbers of ICBMs, the GBI system is not effective, let alone efficient. In reality, however, that is not what it is meant to do, or what it needs to do to achieve its objectives vis-à-vis North Korea.

The (potential) ICBMs of North Korea are pure weapons of terror: if they ever materialise, they will be too few in number and far too imprecise to deliberately destroy specific US military targets, even if carrying nuclear warheads. As such, their main role lies in deterrence and coercion: to present a limited but credible threat against the US homeland that North Korea could use to counter-balance the threats to itself that are inherent in US extended deterrence guarantees to South Korea and Japan.

What GBI is ultimately for, then, is to de-value and discredit that North Korean threat so that Pyongyang cannot rely on it for its coercive potential, so that the US government is not deterred from supporting its allies, and so that Japan and South Korea do not begin to doubt US extended deterrence guarantees as some European allies did after the emergence of the Soviet ICBMs in the 1960s.

If GBI is to be effective, it must be effective first and foremost in the minds of decision-makers in Pyongyang, Seoul, Tokyo and Washington. Maybe GBIs would fail to intercept a North Korean missile, maybe they wouldn't — it is impossible to provide absolute certainty because the event cannot be tested. Having to throw large numbers of mediocre interceptors at any incoming missile is costly and hardly elegant, but it could well be enough to throw doubt on the effectiveness of North Korea's ICBM force as a credible counter-deterrent.

It is impossible to know whether Kim Yong Un's current antics would have been any more dangerous, or whether Japan and South Korea would be any more nervous, or the US government any less willing to demonstrate support to its allies, if US missile defences didn't exist. As with any deterrence strategy, success remains inconclusive and only failure would be obvious. Insofar as US missile defence is first and foremost a tool of deterrence, the same is also true for the effectiveness of the GBI interceptors.

Photo by Flickr user US Missile Defense Agency.