Key point: Millions would die.

Dozens upon dozens of missiles take flight.

For years, the world had heard warnings, but most doubted the day would ever come. Most fall before allied defenses, but one missile finds its mark — it’s the one that matters most. In a flash, hundreds of thousands, if not millions of people perish.

Would North Korea fire off a nuclear weapon? No one knows for certain, but what we do know is that the above scenario is exactly what an aggressive and increasingly-powerful North Korea has been threatening for decades. While the reclusive regime may have previously lacked the necessary weaponry, the North now has the kind of capabilities to turn at least some of its threats into promises.

The U.S. and its East Asian allies have strategic defense assets in position, but war is full of uncertainties. “People think missile defenses are a magic wand. They aren’t,” Jeffrey Lewis, a renowned arms expert, told The Daily Caller News Foundation (TheDCNF).

Here’s what happens if the North pulls the trigger.

What Would Happen If A Launch Appeared Imminent?

The U.S. and its allies in the region are by no means unprepared for a North Korean nuclear attack.

The U.S. and South Korea both have preemptive strike plans for a situation in which a North Korean nuclear attack appears imminent, and while Japan is considering new options, it still relies heavily on U.S. defense.

South Korea has a three-stage defense system, the first stage of which is a preemptive strike option designed to eliminate the North’s offensive capabilities. The “Kill Chain” preemptive strike system detects signs of an impending nuclear missile launch and strikes the North’s nuclear weapons sites and missile bases with cruise missiles and other weaponry.

The U.S. and South Korea also have a joint response plan, Operations Plan (OPLAN) 5015.

While the specifics for OPLAN 5015 are classified, the plan is believed to consolidate previous contingency plans, specifically OPLAN 5029 (internal instability in North Korea), OPLAN 5027 (preparations for an all-out war), and a peacetime plan involving localized provocations from North Korea. OPLAN 5015 is suspected to call for preemptive strikes on the North’s essential military facilities and weapons, and possibly North Korean leadership.

In the event that a nuclear missile strike appeared imminent, allied forces might attempt to eliminate the North’s missiles at launch. Mike Mullen, the former chairman of the Joint Chiefs of Staff, said last year that the U.S. could move to “take out launch capabilities on the launchpad” if North Korea appeared poised to launch a nuclear armed-missile.

The U.S. and South Korea regularly train for such contingencies. For example, during the annual Key Resolve and Foal Eagle exercises, U.S. and South Korean troops practice a “4D” operational plan which involves preemptive military options to detect, disrupt, destroy, and defend against North Korean strikes. The focus is precision strikes on the enemy’s core military facilities and weapons systems.

The challenge is that more and more of North Korea’s missiles are on mobile launchers and scattered about the country. Furthermore, the Korean People’s Army (KPA) has started using solid-fueled missiles, which require significantly less preparation time as they can be fueled in advance and need only a limited crew. Solid-fueled missiles can be fired with less warning and are much harder to track, making them less vulnerable to preemptive strikes.

Another issue is that preemptive strikes on North Korea would be much harder to justify diplomatically, especially if war breaks out in the aftermath, which is practically guaranteed.

What If The Nukes Are Already In The Air?

If a North Korean missile makes it into the air, there are plans for that situation as well.

South Korea and Japan rely on tiered missile defense.

Stage two of South Korea’s three-stage defense system is the Korea Air and Missile Defense (KAMD) system, which is designed to intercept incoming missiles. The U.S. is bolstering South Korean defense through the deployment of a THAAD battery to South Korea, a process that began after North Korea fired four missiles into the Sea of Japan a few weeks ago.

THAAD’s Army Navy/Transportable Radar Surveillance (AN/TYP-2) X-band radar can be configured to one of two settings: forward-base mode and terminal mode. In the latter, the radar has a range of several hundred miles and can facilitate the elimination of missiles in the terminal phase of flight. In the former, the radar’s range is extended, making it possible for THAAD to target projectiles in the initial or launch phase.

To ease China’s concerns about the radar’s ability to peer into its territory, the U.S. has agreed to configure THAAD in terminal mode. China continues to express opposition to the deployment.

THAAD is an important step for South Korean missile defense.

“THAAD is better than anything South Korea has or will have for decades,” Bruce Klingner, who specializes in Korean and Japanese affairs as the senior research fellow for Northeast Asia at the Heritage Foundation, told TheDCNF, “It is imperative that we deploy it to augment the defense of Korea and the U.S. forces deployed there.”

There are also a number of Aegis destroyers operating in the waters off of South Korea. The U.S. has several in the region; Japan has six, and South Korea has three. The Aegis ballistic missile system can track multiple missiles simultaneously and intercept enemy projectiles as needed.

There are certain gaps in South Korea’s defense though. For starters, South Korea’s KAMD is not incorporated into the broader allied defense system, thus weakening its overall effectiveness. Also, the South is particularly vulnerable to submarine-launched ballistic missiles, which the North successfully tested last year.

Japan is much more “forward leaning” in its defense, Klingner notes. Japan has Patriot Advanced Capability (PAC)-2 and 3 systems, Aegis destroyers and SM-3 interceptors, and Japan is considering deploying THAAD and Aegis Ashore units on Japanese soil to boost national defense.

The U.S. has ground-based midcourse defense systems in Fort Greely, Alaska and Vandenberg Air Force Base, California.

Anti-missile systems have their limitations though.

Klingner remarked that “certainly, there is that possibility” that a nuclear-armed ballistic missile could slip through allied defenses, especially given that most regional missile defense systems have never been tested in actual battle conditions.

“Missile defenses help reduce the threat, but they can’t eliminate it,” Jeffrey Lewis, the director of the East Asia Nonproliferation Program in the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey, commented.

In the simplest of terms, missile defense involves hitting a bullet with another bullet, which is no easy task.

“Missile defense systems will never provide 100 percent effectiveness … The addition of THAAD does not guarantee the protection of Seoul, but it does add another piece to the constantly changing puzzle of defense,” Rodger Baker, Vice President of Strategic Analysis at Stratfor, a geopolitical intelligence platform, told TheDCNF.

North Korea is rapidly developing the capabilities necessary to skirt allied ballistic missile defense systems.

“North Korea can probably build missiles (especially ER Scuds) faster and more cheaply than we can build and deploy defenses,” Lewis noted. In recent weapons tests and military drills, North Korea has practiced firing off multiple missiles in rapid succession or simultaneously to overwhelm enemy missile defense systems.

“This is a tactic called ‘salvo fire,’ which is designed to place greater stress on all types of ballistic missile defenses. I don’t know how many simultaneous attacks it would take to ‘saturate’ the battle-management systems in use today by the U.S., South Korean, or Japanese militaries, but the North Koreans seem determined to refine their salvo capabilities,” Joshua Pollack, editor of The Nonproliferation Review and senior research associate in the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey, told TheDCNF. “Even if it didn’t succeed in saturating the defenses, it would at least more rapidly deplete the defense, which has a limited number of shots.”

“Enough simultaneous launches could overwhelm the THAAD system and increase the risk of a nuclear-armed ballistic missile reaching its target in South Korea,” Kelsey Davenport, director of Nonproliferation Policy at the Arms Control Institute, told TheDCNF after North Korea launched three missiles at the same time last September.

The simultaneous launch of multiple missiles is “basic missile defense countermeasure,” Lewis told CNN. “One THAAD battery is not enough. We need at least two, if not many more,” he told TheDCNF.

“The good news is that if defenses hold up against the first salvo, it’s much easier to locate mobile missiles after they fire than before,” Pollack explained, adding, “Ballistic missiles are very hot and bright upon launch, so the point of origin can be detected by satellites very rapidly. Perhaps the empty North Korean missile launch vehicles could be hunted down before they have the chance to reload,” but there is no guarantee.

What Would Be The Post-Launch Reaction?

Many people assume that in the event that North Korea carried out a nuclear strike, successful or not, the U.S. would use nuclear weapons to turn the North into a crater.