The GMD has the ability to “shoot, look, shoot” against an ICBM warhead coming from North Korea. It could launch one GBI, look to determine whether that missile intercepted the warhead, and if it missed, shoot again. If the first GBI succeeds, game over, and Seattle wins. If it misses, however, the GMD would presumably launch a salvo of three more GBIs to have a 91 percent of destroying the warhead. (Note that this assumes there is no systematic problem in the GBIs. But if the first GBI missed the target due to a design flaw or aging component that is present in other GBIs, then the probability of the salvo of three succeeding could fall below 91 percent.)
The problem becomes more complicated if the North Koreans develop decoys. Discriminating between a warhead and decoy poses a major challenge for the GMD, and tests to date involving decoys have not been realistic.
If the North Koreans arm their ICBM aimed at Seattle with five decoys in addition to the warhead, and U.S. space- and ground-based sensors cannot tell which is which, the GMD will have to launch GBIs against each of the six targets as if it were the warhead. If the first salvo does extraordinarily well, each GBI intercepts and destroys its target, one of which is the real warhead (unlikely but possible). If, on the other hand, all six miss (also unlikely but possible), then eighteen more GBIs have to be launched to have a 91 percent chance of saving Seattle from a bad day. In all likelihood, some of the first GBIs would hit their targets, and some would miss. If half hit their targets, then only nine more GBIs would need to be launched in the second salvo against the remaining three targets. All three would have to be targeted, since it would not be known whether the three targets intercepted by the first salvo included the real warhead. (As above, this assumes that there is no systematic problem with the GBIs.)
In the worst case described above, defeating one North Korean ICBM with a warhead and five decoys would exhaust more than half of the current inventory of GBIs. Even in the best case, six interceptors would have to be launched. A GBI costs about $65 million. Thus, in the best case, destroying one warhead would cost $390 million in U.S. military hardware. In the worst case (all six GBIs miss their targets in the first salvo), a total of twenty-four GBIs would be expended at a cost of $1.56 billion—and a chance would remain that Seattle gets incinerated.
To be sure, saving Seattle would be very much worth $1.56 billion, but the problem is that the North Koreans, to say nothing of the Russians and Chinese, can build many warheads and decoys for the equivalent of $390 million, not to mention $1.56 billion. This is why missile defense currently loses the offense-defense competition. Defense may not lose this competition forever. For example, ground-based directed energy weapons capable of firing quickly, accurately and repeatedly might change the equation. However, such weapons are many years in the future. For now, defense loses.
This is not an argument to build no missile defenses against strategic missiles. It makes sense to have some protection in order to deny North Korean ICBMs a “free ride.” But building the capability to intercept more than a handful of ICBMs carrying warheads and decoys would, with current technology, prove expensive—likely prohibitively so, especially in an era of tightening defense budgets.
A New Wrinkle
In addition to GBIs to defend the homeland, the U.S. military has other missile interceptors designed to engage and destroy intermediate- and shorter-range ballistic missiles. The Missile Defense Agency lately has sought to develop a layered defense for the United States, in which GBIs could be backed by Aegis SM-3 Block IIA and THAAD (Terminal High Altitude Area Defense) interceptors. The SM-3 and THAAD were developed originally to defend against ballistic missiles of less than strategic range. However, in November 2020, the Missile Defense Agency successfully tested an SM-3 Block IIA against an ICBM warhead-class target. One test does not establish a reliable capability, but it got the attention of Moscow and Beijing.
The U.S. Navy currently has some fifty warships that are Aegis ballistic missile defense capable, though these ships have multiple missions, and a number are based at foreign ports. However, if the SM-3 IIA proves reliably capable of intercepting ICBM warheads, permanently stationing three or four Aegis-equipped warships armed with SM-3 IIA interceptors off of the eastern, western and Gulf coasts of the United States or deploying SM-3 IIA interceptors on U.S. territory could provide an additional degree of protection against ICBM warheads for the forty-eight continental states—or at the least could be seen as doing so in Moscow and Beijing.
If further tests establish a reliable capability on the part of the SM-3 Block IIA and/or THAAD to intercept ICBM warheads, that would dramatically increase the number of interceptors that could engage strategic ballistic missile warheads. In contrast to the planned sixty-four GBIs by the end of the decade, the U.S. Navy intends to deploy a much larger number of SM-3 IIAs. It expects to procure fifty SM-3 IIA interceptors per year beginning in fiscal year 2024.
By one estimate, the SM-3 IIA will have a unit cost of $18.4 million; the Congressional Budget Office puts the unit cost at $30 million. Either would make the SM-3 IIA less expensive than the GBI, but the price would still likely well exceed the cost of warheads or decoys that potential adversaries could add to their strategic ballistic missiles to overwhelm the defense.
If the Biden administration considers that limiting all Russian nuclear warheads is worth accepting some limits on missile defense, it should factor in that giving SM-3 IIAs and THAADs the capability to intercept ICBM and SLBM warheads could make it extremely difficult to find a mutually acceptable deal. Even if SM-3 IIAs and THAADs in the future were not tested against ICBM warhead-class targets, U.S. officials might face the requirement for some revival of the 1997 ABM Treaty Protocol on defining the difference between strategic and non-strategic missile defense interceptors along with a limit on GBIs.
Downsides to Limiting Missile Defense
Limiting missile defense would have downsides. First of all, it would constrain the U.S. capability to protect against North Korean strategic ballistic missile attack. That said, while the possibility that its ICBM warhead(s) would be intercepted by U.S. missile defenses would likely reinforce Kim Jong-un’s reasons not to launch, the greatest impact in deterring him would surely come from the certainty that, if he attacks the United States or a U.S. ally with a nuclear weapon, his regime would be obliterated in a matter of days, if not hours.
Second, agreeing to limit missile defenses could well reduce the momentum to develop technologies that in the future could prove promising, either kinetic interceptors or directed energy, that not only work well but are cost-effective. Such missile defenses could change the offense-defense calculation by making defense far more competitive than at present, perhaps even against a large strategic ballistic missile attack by a peer competitor.
Third, if the Biden administration chooses to accept limits on missile defense, it will have to work hard to sell that to Congress. In particular, some Republicans seem ideologically committed to missile defense, regardless of the success or failure of systems when tested.
Questions to Consider
In the late 1960s, the U.S. military developed multiple independently-targeted reentry vehicles (MIRVs) for its ICBM and SLBMs, giving its strategic missiles the ability to target three to fourteen separate targets. Nothing indicates that much thought was given to the consequences. One result: in the 1970s, the Soviet military developed MIRVs for its large ICBMs, and, for the next twenty years, the primary worry of U.S. nuclear strategists was a Soviet first strike with those MIRVed ICBMs that would decimate the U.S. ICBM force.
The Biden administration should not let missile defense programs move forward on auto-pilot but consider the following questions as it decides how to proceed:
What are the prospects that GBIs and other missile interceptors will significantly improve their capabilities against ICBM-class targets over the next ten-to-fifteen years? What are the prospects of developing capabilities to discriminate between warheads and decoys?
How important for U.S. and allied security are limits on and reductions in all Russian nuclear weapons, including non-strategic nuclear arms? Does that interest justify countenancing some constraints on missile defense?
What steps might Russia take if it believes the United States seeks a large-scale missile defense to defend the American homeland? What would that mean for a New START follow-on treaty? Could missile defense concerns lead Moscow to conclude that its strategic offensive forces should no longer be bound by any treaty limits?
What steps might China take if it believes the United States seeks a large-scale missile defense to defend the American homeland? If China decided to significantly expand the size of its strategic forces, how would that affect U.S. security calculations?
If the United States does not intend to reorient U.S. Aegis-capable warships to missile defense of the homeland and does not plan to operationally deploy SM-3 Block IIA or THAAD interceptors on U.S. territory, what are the advantages and disadvantages of establishing that those interceptors have the ability to engage ICBM warheads?