A South Korean man watches a TV news program showing an image published in North Korea's Rodong Sinmun newspaper of North Korea's ballistic missile believed to have been launched from underwater. Ahn Young-oon/AP

"North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the US," President-elect Donald Trump tweeted on January 2. "It won't happen!"

However, the terrifying truth is that North Korea, the only country to have tested nuclear weapons in the 21st century, has just as much of a say in whether its potential nuclear arms can or will reach the US as Trump and the US do.

"It can be difficult to make assessments about North Korea's nuclear capabilities given that we have very little access to North Korea's missile facilities," Kelsey Davenport, the director of nonproliferation policy and a North Korea expert at the Arms Control Association, told Business Insider.

"But it's clear that North Korea has made significant advances both with nuclear warheads and with ballistic missiles," Davenport said.

North Korea's nuclear arsenal is still in its early phases, but Kim Jong Un, the country's leader, commands about 100 missile launchers with several missiles for each, according to Jeffrey Lewis, the founding publisher of Arms Control Wonk.

While there's some debate about North Korea's stockpile of nuclear materials, "you're looking at a few tens of warheads, but that number's going to keep going up every year," Lewis told Business Insider.

In comparison, the US has 1,796 nuclear missiles deployed, another 4,500 stockpiled, and 2,800 retired and waiting to be dismantled, according to the Arms Control Association.

Furthermore, North Korea presently has no way of reaching any part of the US with a missile of any sort, but Pyongyang is "likely at the point now where it could mount a nuclear warhead on a medium-range missile, and that would put South Korea, Japan, and US military installations in range of the North Korean nuclear threat," Davenport said.

North Korean leader Kim Jong Un. Reuters

North Korea is a tiny, poor, backward nation with limited missile capabilities and a small nuclear stockpile, but it poses a very serious threat to the US and its allies. Ultimately, there's extremely little the US could do to stop the rogue nation should it chose to strike.

A THAAD battery fires an interceptor. Missile Defense Agency

Could the US stop a North Korean nuclear attack?

It's complicated.

The US and its allies have three major forms of missile defense against North Korea.

"Missiles come in a variety of ranges," Lewis said. "Every missile defense system is set to deal with a small subset of missiles in a particular range and at one stage in flight."

For the short- and medium-range missiles with which North Korea could look to strike a nearby foe — or the 25,000 US troops stationed in South Korea — the US has Aegis radar-equipped Navy destroyers.

"That's good for medium-range missiles," Lewis said.

Next, Patriot Advanced Capability-3 interceptors defend against missiles at their final, or terminal, stage. These are "mostly good at short- and medium-range ballistic missiles," said Lewis. The PAC-3 "would cover a city or an airfield," he added.

Finally, the biggest and perhaps best system is the Terminal High-Altitude Area Defense system. "THAAD could cover all of South Korea, including everything up to a Nodong missile," North Korea's medium-range system, Lewis said.

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On maps and graphics, you can see the overlapping areas of protection provided by these three systems, but looks can deceive; with missile defense, all systems deal in probabilities, not certainties.

"You actually have to see the thing get launched, understand where it's heading, and pass the information along to battlefield management software, the brain that makes all of this work," said Lewis.

Of all the steps in the process, not one is immune to distortion.

"Radars can be defeated by chaff or clutter," Lewis said. "North Korea could launch a radar blackout attack, where a nuke detonates in the atmosphere and can black out a radar for a few seconds. Those could be the few seconds you need."

"Missile defense is not a surefire way to negate the threat posed by another country's nuclear-capable ballistic missiles," said Kelsey Davenport. Agence France Presse

Davenport said, "It's important to note that this THAAD system will only cover North Korea, but North Korea could evade that by launching a nuke from a submarine from outside of THAAD radar."

And North Korea may "try to confuse the THAAD system by launching multiple missiles at once or launching decoys," she said.

But could North Korean missiles overwhelm the US's defenses in its homeland? Experts say we have every reason to believe Kim when he says he's working toward an intercontinental ballistic missile, and the US's defenses suffer from the same uncertainties as systems abroad.

The US protects its western coast from a fixed site in Alaska, where interceptor missiles would theoretically strike an incoming ICBM "midtrajectory, while it's traveling through space," Lewis said.

However, as a recent Bloomberg article noted, the office that tests this system concluded it had "limited capability to defend the US homeland from small numbers of simple" ICBMs, according to its last report.

Lewis echoed this, saying it has "a spotty test record" and that there are multiple questions about how well it would perform. Unlike THAAD, the Alaska site fires salvos, a series of interceptor missiles for each incoming threat, which serves as an admission that the system falls short of perfection, according to Lewis.

"The system in Alaska needs to be redesigned," Lewis said. "They plan to salvo-fire it, so every interceptor has a 50-50 chance of hitting. ... If they fire five, they're gonna be up in the high-confidence territory" for intercepts.

But this high ratio of interceptors to threats means that a North Korean salvo could possibly exhaust the US's supply of interceptors with decoys, leaving the US defenseless.

So for now, the only guarantee the US has against North Korean ICBMs is that such a threat doesn't exist.

So why doesn't the US just destroy North Korea's nuclear capabilities?

Each day, North Korea gets closer to issuing a real threat to the US, and it already significantly endangers the lives of millions within its range. Yet the US can't exactly swoop in and stop it.

"A preemptive strike on North Korea would carry an enormous risk of retaliation on South Korea or US assets in the area," Davenport said.

"The big dynamic that's a problem is that, I think, North Korea plans to use those missiles armed with nuclear weapons at the early part of the conflict to destroy US forces in the region and those coming in," said Lewis, adding that Kim's strategy would likely be to "impede an invasion and shock us."

"The problem the US and South Korea faces is that the options for defense are not all that appealing," said Lewis.

US pilots currently train in mock North Korean airspace with stealth planes like the F-22 and F-35 to destroy surface-to-air missiles, or SAM, and nuclear sites. While the fifth-generation aircraft would likely succeed and overwhelm North Korean forces, the nuclear sites are just too spread out and mysterious to knock out before Pyongyang would have a chance to strike back.

An F-22 Raptor flies in formation behind a KC-135 Stratotanker. US Air Force

"There are so many unknowns about the number of warheads North Korea has, where it stores them. ... It would be incredibly difficult to ensure that a preemptive strike would neutralize the North Korean threat or even the conventional threat posed to Seoul," said Davenport, alluding to the huge artillery installations North Korea has fixed on the South Korean capital that are ready to blast away.

When it comes to using jets to hunt down SAM and nuclear sites, "the US tried this in Iraq in 1991, and it was a total failure," Lewis said. The US's considerable losses of aircraft to antiaircraft batteries during the Gulf War was a "searing experience for the US Air Force," Lewis said.

While there's plenty of reason to think that today's F-22s and the coming F-35s would far outmatch North Korea's technology and air defenses, the terrain of North Korea plays well for Kim.

Iraq's countryside is defined by flat desert expanses, where road-mobile antiaircraft batteries can easily navigate but have nowhere to hide. North Korea, on the other hand, has mountains and forests, though the country is smaller — therefore, the road-mobile missile launchers and antiaircraft batteries would have more opportunities to hide but less space to do so.

In any case, the landscape presents difficulties in hunting down sensitive sites, even with the best jets the US has to offer.

The landscapes of Iraq and North Korea couldn't be more different. Wikimedia Commons

And unlike the US, North Korea has road-mobile missile launchers that can hide anywhere.

"One hundred launchers, so it would be a pretty big lift, and you have to do it pretty fast" to avoid a North Korean counterattack, said Lewis. Many have suggested that instead of disarming North Korea with a lightning-quick blitz from the air, forces decapitate the regime by striking Kim himself.

In fact, South Korea recently announced plans to form a small "decapitation brigade" that would surgically destroy the leader and his top leadership — but that's a best-case scenario.

The terrifying truth about North Korea's nuclear threat is that it can't be stopped by one system or even multiple systems. It can't be blitzed from the sky. It can't be effectively debilitated by sanctions, as time has proven, and it only strengthens over time.

Several possible solutions circulate in the national-security arena, all with strengths and weaknesses, all risking innocent lives. And each side appears set on striking first and ending the conflict before it begins.

"The US and North Korean war plans are to go first," Lewis said. "South Korea plans to go first. All three independent parties plan to go first, and two of them are wrong. It's a dangerous situation people haven't thought through."