U.S. President Barack Obama’s decision to beef up the nation’s territorial missile shield could reassure U.S. allies who feel increasingly threatened by North Korea’s bellicose behavior while blunting congressional critics who want to see more investment in missile defense.

But the protective rationale for installing 14 new interceptors in Alaska, at a cost of some $1 billion, is not convincing, unless one believes North Korea will have the capability — and the inclination — to mount an attack involving a dozen or more nuclear-tipped ICBMs at the United States within the next decade.

The current U.S. territorial shield, known as the Ground-based Midcourse Defense (GMD) system, includes 30 interceptors: 26 at Fort Greely, Alaska, and four at Vandenberg Air Force Base, Calif. The system is intended to defend against a limited, desperation strike by a so-called rogue state like North Korea.

Though the system has a spotty track record in testing, it’s probably fair to assume that it provides some degree of protection — something being better than nothing at all. But setting aside the nagging question of whether North Korea’s government — or Iran’s, for that matter — would even contemplate launching an attack that would ensure its own demise, it is far from clear how increasing the number interceptors along the U.S. Pacific Coast from 30 to 44 will provide a commensurate degree of added protection.

There of course can be little doubt that North Korea represents a threat to the United States and some of its allies, most notably South Korea and Japan. The danger is measurable in the fact that Pyongyang in the last several months has launched a satellite into orbit, strongly suggesting it is close to mastering ICBM technology, and detonated a nuclear device for the third time. It is only prudent to assume North Korea is working on putting the two technologies together and eventually will have a long-range nuclear-strike capability. It’s also true that diplomatic efforts to curb North Korea’s nuclear ambitions, or to get Pyongyang to behave somewhat responsibly, have failed miserably.

But to the extent that the GMD system works, 30 interceptors should be more than sufficient to address any realistic near- or medium-term attack scenarios involving North Korea. As for long-term protection, the Pentagon would be better off investing its increasingly scarce resources in qualitative rather than quantitative improvements, such as better discrimination between targets and decoys, new kill vehicles or higher-speed interceptors.

To help cover the cost of its GMD expansion, the White House will divert funds from an effort to develop a new interceptor dubbed Standard Missile 3-Block 2B, which was intended for installation in Europe to help protect U.S. territory from an Iranian strike. While it’s difficult to envision a scenario under which Iran would attack the United States, the Islamic republic’s demonstrated satellite launching capability, coupled with its nuclear ambitions, is cause for grave concern.

According to the U.S. Government Accountability Office, however, Europe-deployed Block 2B interceptors would not effectively enhance the U.S. territorial shield against Middle Eastern threats. That report, which cited the Pentagon’s own analyses, echoed a National Research Council study released last fall that said a much better option would be to place high-speed interceptors at a site near the U.S. East Coast.

Given these findings, the decision to restructure the Block 2B program, which Congress wasn’t supporting in any event, is justified. So are the decisions to install another missile tracking radar in Japan and to assess the environmental impact of a third interceptor site in the United States, possibly along the East Coast, as mandated by Congress.

But expanding GMD interceptor deployments in Alaska won’t address the Iranian threat any better than the Block 2B. The National Research Council report cast serious doubt on the GMD system’s ability to protect the eastern United States from threats in the Middle East. The reasons are related to the distance the interceptors would have to cover, rather than the number available.

The GMD was hurried into place to address what many U.S. government officials viewed as an urgent threat, primarily from North Korea. That threat obviously isn’t going away — in terms of capability, it is growing. But now that a rudimentary capability is in place, the investment focus should be on qualitative enhancements to the existing system and on technologies that could lead to dramatic improvements a decade from now. A nearly 50 percent increase in the number of deployed interceptors provides a measurable indicator of the White House’s commitment to missile defense, but whether that translates into a corresponding boost in U.S. or allied security is another question entirely.