The Trump administration claims “all options are on the table” for dealing with North Korea’s nuclear-weapons program—from using military force, to pressuring China to cut off economic relations with North Korea, to Donald Trump negotiating directly with Kim Jong Un. But what do those options look like? And what consequences could they have? This series explores those questions, option by option.

Trump’s Reddish Line

Millions of lives may depend on what Donald Trump means by the word “it.”

At some point, the American president recently told CBS’s John Dickerson, North Korean leader Kim Jong Un will develop “a better [missile] delivery system” for its small but growing stockpile of nuclear weapons. “And if that happens,” Trump vowed, “we can’t allow it to happen.”

Behind the seemingly contradictory statement was a hazy hint of what might prompt the world’s mightiest military to use force against an emerging nuclear power, potentially drawing China, Japan, and South Korea into one of the most volatile conflicts in living memory.

The next day, on Fox News, Trump’s national-security adviser offered one answer to what his boss meant by “it.” It’s unacceptable for North Korea to obtain the means of hitting the United States with a nuclear weapon, H.R. McMaster told Chris Wallace, citing a scenario that, according to many experts, could become a reality within Trump’s first term in office. On Sunday, North Korea made a major advance toward that goal by testing a missile that may be capable of reaching U.S. military facilities on the island of Guam and, if you believe the North Korean government, carrying a nuclear warhead.

Wallace mentioned the concern most people raise when they assess the risks of a U.S. military operation: The North Koreans could swiftly bombard South Koreans with artillery guns stationed in the Kaesong area near the Korean Demilitarized Zone (DMZ)—roughly as far from the South Korean capital of Seoul, a sprawling metropolis of 25 million people, as Baltimore is from Washington, D.C. “What the president has first and foremost on his mind is to protect the American people,” McMaster said. Where that left the South Korean people, McMaster didn’t say.

“If we were to launch a preemptive strike against their nuclear program, their missile program, we’re talking about human catastrophe, aren’t we?” Wallace inquired.

“Well, yes,” McMaster replied.

The Options Within the Military Option

A “military” option need not involve strikes at all. It could mean deploying military assets to deter North Korea from using weapons of mass destruction, just as the United States and its allies deterred rival nuclear states during the Cold War. This might include beefing up missile defenses like the THAAD system in South Korea, which the Trump administration has been rapidly installing. It might even include “the reintroduction of nuclear weapons” to Japan and South Korea to emphasize “that we’re determined to fully deter any [North Korean] attack and, should deterrence fail, that it would result in a prompt and overwhelming response,” just as America deployed Pershing missiles to Europe during the Cold War, according to Wallace Gregson, a retired lieutenant general who served as the assistant secretary of defense for Asian and Pacific security affairs in the Obama administration.

But the Trump administration also appears to have not ruled out a “preventive strike” akin to what the George W. Bush administration undertook in Iraq—striking first to neutralize a threat that may materialize in the future, according to Van Jackson, a Korea expert at Victoria University of Wellington. (A “preemptive strike,” by contrast, would be the U.S. government springing into action to stop an imminent attack.)

As Jackson sees it, preventive strikes against North Korea could take three forms:

1) The offshore option: The United States launches Tomahawk cruise missiles from a Navy ship or submarine. This is the least risky option since it doesn’t involve traversing North Korean territory, and would resemble Trump’s strikes against the Syrian military for using chemical weapons, but there’s no guarantee that North Korea would refrain from retaliating.  

2) The aerial option: U.S. stealth bombers or fighter aircraft conduct air strikes over North Korea. Such an approach is more conducive to military escalation than the first option. The North Korean government knows that U.S. bombers are capable of carrying nuclear weapons, and the United States might need to take out North Korean air defenses to successfully deploy some of these planes. Even if the U.S. operation is limited in scope, North Korea may not interpret it that way.

3) The high-tech options: A U.S. bomber drops a Massive Ordnance Penetrator, a bunker-busting bomb that has never been used in combat, on hard-to-reach targets such as underground nuclear facilities. Or the U.S. trots out other new technologies, such as electromagnetic railguns mounted on warships. Using these weapons would set a precedent that other countries could emulate, and it’s unclear whether they would be any more effective than lower-tech options.

Depending on the military assets used and the purpose of the strikes, the Trump administration could hit a variety of targets, including:

1) Facilities for producing and storing nuclear material and nuclear weapons

2) Facilities for producing and storing missiles

3) Missile launchers, particularly North Korea’s expanding fleet of mobile platforms

4) Ports for submarines capable of launching missiles

5) Artillery positions near the DMZ that could be used in a retaliatory attack

Since these targets are dispersed across the country and often concealed underground or undersea, however, U.S. strikes won’t eliminate the country’s nuclear-weapons program, according to Victor Cha, a Korea expert who was serving in the George W. Bush administration when North Korea tested its first nuclear weapon. At best, they’ll set the program back several years.

Daryl Press, a scholar of nuclear deterrence at Dartmouth College, said America’s capacity to inflict severe damage on North Korea’s program shouldn’t be underestimated. But he argued that this capability is best-suited to responding to an outbreak of war between North and South Korea or to North Korea using or threatening to imminently use nuclear weapons, rather than as a preventive measure.

“If there were a conventional war on the Korean peninsula, and thousands of people are dying on all sides, it might be very valuable for the United States to rapidly degrade North Korea’s nuclear forces and also its artillery,” Press explained. A conventional war might encourage the North Koreans to use nuclear weapons, since the U.S. and South Korean militaries are far stronger than North Korea’s and would triumph in a normal conflict. (A 2017 ranking of military powers puts the United States first and South Korea 11th, while North Korea places 23rd.) “If North Korean nuclear escalation seemed likely, even a 90 percent or a 95 percent successful attack on the North Korean delivery systems would be a great victory, because every weapon destroyed is one that’s not detonating in South Korea or Japan.”

But in the absence of war, Press continued, each weapon destroyed is not necessarily a victory: “In peacetime … even largely successful strikes against North Korea’s nuclear program or its conventional artillery could very well leave residual North Korean capabilities that could cause incredible amounts of damage and suffering in South Korea and possibly Japan.” Destroy 90 percent of North Korea’s firepower, and you and your allies still have to reckon with the remaining 10 percent.

How Would North Korea Respond?

Kim Jong Un, like his father Kim Jong Il, appears to view nuclear weapons as the most reliable way to deter foreign aggression. “The Saddam Hussein regime in Iraq and the Gaddafi regime in Libya could not escape the fate of destruction after being deprived of their foundations for nuclear development and giving up nuclear programs of their own accord,” the state-run Korean Central News Agency solemnly observed in 2016.

Given this, Press reasoned, North Korean leaders could interpret any U.S. attack on their nuclear infrastructure as a prelude to invading or overthrowing the government, even if the United States insists otherwise. To force the United States and its regional allies to back off, the North Koreans might carry out conventional attacks on Seoul and U.S. military bases in Japan and South Korea, unleash chemical weapons on those targets, or even make use of whatever nuclear weapons survive the initial U.S. strikes. (It’s unclear whether North Korea has the ability to place a nuclear warhead on a missile that can reach Japan or South Korea, but many experts suspect they do.)

Of course, any of these audacious actions would likely invite an enormous U.S. and South Korean military assault on the Kim government. But the limited deployment of nuclear weapons to spook adversaries into a ceasefire—in other words, escalating to de-escalate—was actually part of NATO’s strategy against the superior conventional forces of Warsaw Pact countries in Europe during the Cold War, according to Press.

“Political scientists expect generally that if you have only a small number of nukes, to be able to squeeze political value out of them at all, you need to be able to use them early” in a conflict, Jackson noted.

In the 1980s and ’90s, the image that haunted people contemplating military conflict with North Korea was that of North Korean artillery setting Seoul ablaze, Jackson said. The image endures today, as Chris Wallace’s interview with H.R. McMaster attests. But it is outdated (and unrealistic, since the North would only be able to fire for so long before facing massive counterfire).

In building out its nuclear arsenal, Jackson argued, North Korea is extending its “escalation ladder,” freeing up lower rungs for military maneuvering that might not spell the end of the Kim government. North Korea, for example, could respond to U.S. strikes by launching non-nuclear missiles at the South Korean port of Busan, making it harder for the United States to deploy forces in the region. At the same time, it could threaten to attack Seoul or use nuclear-tipped missiles against South Korea or Japan if the U.S. retaliates for the Busan assault. “They have the ability to hold things at risk while attacking in ways that they could not have before,” Jackson said. (As Press and co-author Keir Lieber wrote in a 2013 Foreign Affairs article, “The key to coercion is the hostage that is still alive.”)

Since the 1960s, the North Korean military’s “theory of victory”—“how they believe military force can achieve political goals and coerce successfully and control escalation, etc.”—has been “highly offensive,” Jackson said. North Korean leaders appear to believe that they have to periodically engage in low-level violence to achieve high-level deterrence, and that they must reflexively reciprocate when attacked.

Consider North Korea’s most aggressive act against the United States since the Korean War: After North Korea shot down a U.S. spy plane in 1969, killing all 31 Americans on board, the North Korean foreign minister recounted the government’s rationale in a conversation with the Soviet ambassador to North Korea. “If the enemy fires on us in [the DMZ] with machine guns we respond with machine guns; when he uses artillery, we also use artillery,” Pak Seong Cheol explained at the time. “When the Americans understand that there is a weak enemy before them they will start a war right away. If, however, they see that there is a strong partner before them, this delays the beginning of a war.”

While the U.S. government has typically responded with restraint to North Korean provocations, U.S. military leaders—who under Donald Trump dominate national-security policy in a way they didn’t under Barack Obama—have a similarly offensive theory of victory, Jackson argued: “They believe that military force has political value, that you have to escalate to de-escalate, and that you can purchase general deterrence through deliberate friction with your adversary—through military posturing and pinpricks.” When two offensive theories of victory collide, and when each is held by a country with an array of fearsome military capabilities, Jackson warned, “It’s very easy to lock into a conflict spiral.”

“It’s certainly true that dictatorships like North Korea—their primary goal is to survive,” Cha told me. “So could you carry out a strike against their nuclear facilities with a threat that if they retaliate we will wipe out the regime? Will a rational dictator then sort of sit still? Possibly. But that’s a big risk to take.” How big? A second Korean war could inflict “millions of casualties,” he said.

Then What?

Sam Gardiner, a retired U.S. Air Force colonel, has been conducting war games involving a Korean conflict for the past 25 years, many while teaching colonels and lieutenant colonels at the National War College.

In a war game organized by The Atlantic in 2005, a year before North Korea tested its first nuclear weapon, experts predicted that, in the event of hostilities with the United States and South Korea, North Korea would draw on its chemical-weapons arsenal, which is one of the largest in the world. The resulting conflict, they speculated, could kill hundreds of thousands of South Koreans and Americans in South Korea—an estimate Gardiner now believes was low given what he’s learned in recent years from the lethality of chemical-weapons attacks in Syria.

Gardiner sent me an essay he’d just composed imagining what war on the Korean peninsula might look like. In the scenario, North Korea conducts a ballistic-missile test and its sixth nuclear test on the same day, prompting the Trump administration to respond to the provocation by launching 20 Tomahawk cruise missiles at North Korea’s Yongbyon nuclear complex.

“The White House meant it to be a limited response with a clear message,” Gardiner writes. “The United States targeted the North Korean nuclear program, not North Korea’s nuclear weapons. The nuclear-research center was hit with five weapons, the radiochemistry laboratory was hit, Building 500 close to that laboratory was hit, and the fuel fabrication facility was hit. The targeting was done carefully to avoid the nuclear-research reactor, the experimental reactor, as well as the nuclear power plant.”

Gardiner envisions panic in Seoul as civilians are evacuated and an eerie lack of response for 48 long hours from North Korea, which is covertly inserting thousands of special-operations troops into South Korea via small boats, light aircraft, and hidden tunnels under the DMZ. The soldiers eventually emerge, striking U.S. and South Korean air bases in South Korea with conventional weapons and sarin gas, and kidnapping a number of American, Japanese, and South Korean officials. Seoul soon comes under fire. North Korean conventional ground troops mobilize as the U.S. and South Korean militaries launch a massive military campaign against the North, with the goal of overthrowing the Kim regime and occupying the top half of the peninsula within 60 days.

“We are less than 24 hours into the battle,” Gardiner writes. “The medical situation in Seoul is in crisis. Some estimates have put the casualties from conventional shelling and chemicals at over 1 million. It will be a long time before we really know.”

What Does the U.S. Actually Want?

One of the lessons Gardiner learned from his war games: “If you don’t know what your objective is, it’s not possible to find military options to achieve it.” And, at the moment, he’s not sure what to make of the Trump administration’s objective for North Korea. McMaster and Vice President Mike Pence have said that America’s goal is to rid the Korean peninsula of nuclear weapons, but Trump and Defense Secretary James Mattis have instead stressed that North Korea isn’t behaving, “as if there is some form of behavior that we expect them to achieve” rather than a renunciation of nuclear weapons. With these shifting messages, Kim Jong Un wouldn’t necessarily know how to avoid a military conflict with the United States even if he wanted to. Which brings us back to what Donald Trump means when he says “we can’t allow it to happen.”

“I’ve lost faith that North Korea will ever voluntarily denuclearize,” Gregson said. And “the question’s gotta be asked: Do we ever get denuclearization without regime change? If not, what are we—we, collectively … our allies South Korea, Japan, and Vietnam—prepared [to do]?”

Even if the Trump administration had a coherent objective, the mission could evolve in the course of a war. “In most [war] games, once there has been an exchange which causes serious casualties in the South, the objective of the South Koreans and the Americans changes, and they begin to think, ‘You have to have regime change,’” Gardiner told me. “Let’s say there are 10,000 Americans killed in just a conventional strike [by North Korea]. The pressure from the American people would be, ‘It’s time to eliminate this guy.’ The casualties force you to lose control of the situation.”

It’s therefore worth asking whether the potential upside of military strikes is worth the potential downside. Is a North Korea with nukes that can reach the United States “really the threat that the administration implies it is?” Gardiner wondered. “There is no reason to believe that a North Korea with nuclear weapons doesn’t end up being stable”—or, at least, as stable as a North Korea with chemical weapons and biological weapons and ballistic missiles and loads of artillery has been for the past several decades.

I asked Gardiner how his Korea war games typically ended. “They ended 60 to 100 days into a conflict where the U.S. and South Korea are beginning to attack towards [the North Korean capital of] Pyongyang,” he said. At that point, U.S. troops are occupying North Korea, they don’t know where the nuclear weapons are, and they’re suddenly responsible for a starving population. “You step back and say, ‘My goodness. What have we done?’”