For decades, US presidents have used diplomatic pressure and economic sanctions to try and convince North Korea to abandon its nuclear program. While doing so they have also been working at home on a Plan B: defense.

The Pentagon has been developing a nationwide antimissile program since the early 1990s. The aim is to protect American territory – not from established nuclear powers Russia or China, but any smaller Intercontinental Ballistic Missile (ICBM) arsenals produced by North Korea, or (possibly) Iran.

Now that nascent missile defense faces an important inflection point, as does the overall effort to block Pyongyang’s nuclear ambitions. Increasingly it seems a matter of when, not if, North Korea will develop the means to target the continental US with a nuclear-tipped ICBM.

That moment might be reached in three to five years, according to current and former US defense officials. And by 2020, North Korea could have as many as 100 nuclear warheads, according to a 2015 Johns Hopkins University report.

At that point, will US missile defense be adequate for its task? Even supporters describe the current system as more of an advanced prototype than a finished product. It might be able to protect against an initial North Korean nuclear capability, but if Pyongyang establishes and maintains serial production of missiles, today’s US defensive capabilities might soon become inadequate.

“We’re not willing to accept a strategic relationship of vulnerability to North Korean missiles, in the way we have, de facto, with Russia and China.... This is important. We have to get this right,” says Thomas Karako, a senior fellow and director of the Missile Defense Project at the Center for Strategic and International Studies (CSIS) in Washington.

More modest than Reagan's 'Star Wars' dream

The US has been working on various anti-missile programs almost since the dawn of the ICBM age. In terms of funding and prominence, this effort perhaps reached its apogee with President Reagan’s “Star Wars” Strategic Defense Initiative. SDI envisioned a multi-layered system able to target and attack ballistic missiles from launch to warhead descent. Today’s deployed system is not nearly as broad as that dream.

The current US missile defense is aimed instead at shielding the nation from nuclear blackmail or terrorism or threats from a rogue state. (Both China and Russia oppose US defenses, saying it is possible they will destabilize the mutual deterrence that currently exists between big nuclear powers.)

On the list of today’s “rogue states”, North Korea sits at No. 1. The US intelligence community assesses that North Korea is currently in the process of fielding an ICBM capability to strike the American homeland with a nuclear warhead. Such a system hasn’t been tested, nor is it clear whether any North Korean ballistic missiles of shorter range have yet been tipped with nuclear warheads.

After all, this is rocket science, meaning very difficult – as Pyongyang’s many failed missile tests show.

Defenses from Hawaii

The first line of US ballistic missile defense is a global network of sea-, land-, and space-based sensors to detect and track any launch against American targets.

These range from an ocean-going X-Band radar at Pearl Harbor in Hawaii, to early-warning radars strung across Alaska, Greenland, Britain, and other northern spots, and SPY-1 radars on Navy Aegis missile defense ships at sea. Data is fed to a central fire control system at Schriever Air Force Base in Colorado Springs.

Since 2004, the US has deployed rocket interceptors at Ft. Greeley, Alaska, and Vandenberg Air Force Base in California. Currently there are 36; that number is scheduled to rise to 44 by the end of 2017.

The three-stage interceptors are intended to target missile warheads in the middle of their ballistic course from launch to target. They carry “kill vehicle” warheads of their own, which separate from the launcher and maneuver towards the coasting nuclear warheads. An upgraded Redesigned Kill Vehicle is in the works. Testing won’t begin for a few years; deployment is currently scheduled for 2020.

Testing record: 9 of 17 attempts successful

The US has some mobile defense assets that can augment this basic system. The Terminal High Altitude Area Defense (THAAD) is a rapidly deployable battery of interceptor missiles designed to shoot down short- or medium-range ballistic missiles in the final stages of its flight. It is intended to protect defined areas, such as cities or military forces, as opposed to entire countries. The US and South Korea have recently set up a THAAD system on a former golf course in South Korea.

The Navy’s Aegis cruisers and destroyers also carry interceptor missiles that are designed to give them the ability to defend regions against short and medium-range missile attack. The Aegis defense has the advantage of easy mobility – but the number of ships is limited, and they sometimes have other missions to fulfill.

Is this integrated system effective? After all, in essence it is attempting to hit a bullet with a bullet – not an easy thing to do. Since 1997, the payload has destroyed its target in nine of 17 full-blown intercept tests, or just over 50 percent of the time.

Some scientists harshly criticize the US missile defense program, saying that interceptors could be easily spoofed.

The ground-based defense system “is not on a credible path to achieving an operationally useful capability,” charged the Union of Concerned Scientists in a 2016 report on the effort.

But officials of the Pentagon’s Missile Defense Agency and other proponents say the system is a capable one that is being refined to meet a threat which itself is still developing. They say its testing record should be seen in that light.

A Congressional Research Service report on the system drawn up in late 2016 attempts to strike a balance between these points of view.

“Although the [ground-based missile defense] system is praised by senior military leaders and is generally viewed in successful terms, it does have a somewhat mixed flight test record,” writes CRS analyst Steven Hildreth.

Alaskan senator pushes for more robust missile defense

Meanwhile, North Korea grinds ahead with its military programs. That is the military and political reality facing the US, note defense proponents. Holding a nuclear threat over the United States seems a core goal of Kim Jung-Un's worldview. Is that a situation the US can endure?

“Each of the last four administrations has looked at the North Korean threat and said this is not the sort of thing in which we can live, in a state of vulnerability,” says Dr. Karako of CSIS, a principal author of a new “Missile Defense 2020” report that urges devoting more money and effort to outpacing the ballistic missile threat.

Among other recommendations, the CSIS study urges fielding upward of 80 ground-based interceptors by 2020, and completing readiness efforts studying a possible East Coast deployment site.

Some lawmakers are already on board. Alaska, closer to North Korea than the lower 48 states, could be an early target for attack. Sen. Dan Sullivan (R) of Alaska says that in his view the US needs to significantly step up its missile defense system. But “nobody’s talking about that,” he said in a Monitor interview last week.

The senator, a member of the Senate Armed Services Committee, says he hopes to soon introduce a bipartisan bill to significantly boost America’s ability to shoot down rogue missiles from North Korea or Iran.

Senator Sullivan proposes 28 more interceptors, as well as requiring the military to study having up to 100 interceptors distributed across the country.

Should North Korea successfully develop a nuclear-tipped ballistic missile, “the pressure on the president will be enormous to do something ‘militarily,’ ” says Sullivan. But if the US has a system that can, with 99.9 percent certainty, shoot down rogue missiles, with the expectation of “massive” US retaliation, then Kim Jong-un will have to “think really hard” about that, the senator says.

“Having a robust missile defense will give the president more options and breathing room,” Sullivan contends.

Cyber sabotage?

But here’s something the Pentagon doesn’t talk about: ramping up investments in interceptor rockets might not be the only US option to blunt North Korean missile development. Secret cyberattacks to disrupt Pyongyang’s missile tests might be an option as well.

In February, The New York Times reported that the Trump administration planned to continue work on an Obama-era program that charged the Pentagon with developing hacking tools to disable or misdirect launched North Korean missiles. That capability, if confirmed, could give the Defense Department a Digital Age tool to deal with the rogue state.

“[Missiles] have to be linked to a network and to a computer. That’s your entry point,” says James Lewis, a senior fellow at the Center for Strategic and International Studies, and a former rapporteur for United Nations cybersecurity talks in 2015. “Breaking into somebody’s weapons systems and trying to interfere with their operations, that’s just part of warfare now.”

Indeed, the US appeared to have expanded its visibility into North Korean computer networks even before the damaging Sony Pictures hack that leaked private emails and the unreleased film The Interview in 2014, which the FBI attributed to Pyongyang’s hackers.

Classified documents disclosed to the press in 2015 indicated that the National Security Agency, with help from US allies in Asia, penetrated into North Korean networks, including devices and systems used by the country’s top hacking teams and spies. The Defense Department could also target North Korea's suspected suppliers, such as Iran, with digital attacks.

But while the Pentagon and other military agencies may be using cyberattacks to probe digitally connected weapons networks, it’s not clear that it has been the driving factor in Pyongyang’s recent spike in failed missile launches.

Even for elite hackers, targeting North Korea's missile program would be particularly complex. Unlike the Stuxnet computer worm – widely believed to have been developed by the US and Israel – that targeted Iran's central nuclear enrichment facility, a digital attack against North Korea's missile program would have to target multiple test sites and mobile batteries that Pyongyang uses to fire missiles.

“Missiles tend to blow up anyway just given how hard rocket science is,” says Adam Segal, a senior fellow at the Council on Foreign Relations. “To do it seconds or minutes after the launch would suggest a kind of pervasiveness in the networks and an all-seeing ability that would be very expensive and very difficult to maintain.”

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Even optimists about using hacking tools against North Korea's missile program see as one piece of a broader solution – not a silver bullet.

"The question is always probability," says CSIS's Mr. Lewis. "If they shot 100 missiles, you could probably disable some of them. You probably couldn't disable all of them."