There can be no debate that North Korea’s nuclear-weapons tests, missile launches into Japanese airspace and threats to attack U.S. territory are hostile acts. What is debatable is what the U.S., Republic of Korea, Japan and other allies in the not-so-pacific Asia Pacific region should do about North Korea.

^{Korean War II}

One option a surprising number of observers (here, here and here) and former and current policymakers (here, here and here) have mentioned in recent weeks is preemptive or preventive military action. Although the terms are often used interchangeably, there is an important difference: Preemptive action is taken in anticipation of an imminent threat and certain attack. Preventive action is taken to prevent a possible attack or emerging threat. Thus, the Iraq War was preventive not preemptive.

To be sure, these sorts of anticipatory military actions are sometimes the least-bad option. Surely, Britain and the world would have been better off had the British government heeded Churchill’s warning from the back bench of Parliament in 1934—when Hitler was still weak—that “Germany is arming fast” and proposing “preventive war to stop Germany from breaking the Treaty of Versailles.” Israel’s 1967 preemptive war on its Arab neighbors as they circled for the kill saved the Jewish state from destruction. Israel’s 1981 preventive strike on Iraq’s nearly-completed nuclear facility at Osirak spared Israel, the Middle East and the United States from a nuclear-armed Saddam Hussein; likewise, its 2007 preventive strike on Syria’s not-yet-completed nuclear facility at Deir el-Zor spared Israel, the Middle East and the United States from a nuclear-armed Bashar Assad.

However, anticipatory action—whether preventive or preemptive—must be weighed against the likely costs. And in the case of the Korean Peninsula, the costs would outweigh the benefits.

President Bill Clinton ordered the Pentagon to develop plans for preemptive action against North Korean nuclear sites. The Air Force even conducted simulated counterproliferation strikes. But those plans were shelved, and understandably so. As the Congressional Research Service concluded, “The tactical success of a counterproliferation mission could be lost in the consequences of another war.”

Indeed, North Korea deploys 13,600 artillery pieces/rocket-launch systems, 4,100 tanks, 730 warplanes and hundreds of missiles. In 2005, Gen. Leon LaPorte, former commander of U.S. Forces-Korea, warned that every third round fired from North Korea’s vast artillery fields would be a chemical weapon. Seoul would bear the brunt of the blow. With its 10 million residents, Seoul sits just 35 miles from the DMZ—a sobering thought given that 70 percent of the North’s military is deployed within 60 miles of the border zone. That explains why experts talk of “World War I levels” of casualties—and why the measure of success in Korea for U.S. presidents is simply getting through another day, another year, another term without another war. It’s a low bar. But given what Korean War II would look like, it’s a worthy goal.

Even a short war—even a war contained to the peninsula—would be ferocious, brutal and bloody. Gen Rob Givens provides some of the details here. The Pentagon has projected more than 200,000 U.S.-ROK military casualties in the first 90 days, along with hundreds of thousands of civilian casualties. As Joint Chiefs Chairman Gen. James Dunford observes, Korean War II would be “horrific…a loss of life unlike any we have experienced in our lifetimes.”

Worse, none of this takes into account the specter of nuclear weapons being used against the ROK, Japan, the U.S. and/or North Korea. Moreover, Korean War II would directly impact four of the world’s largest economies representing almost 50 percent of global GDP (the U.S., ROK, Japan and China).

Such a war would give new meaning to the term “Pyrrhic victory.”

Given the high risks, the low tolerance for casualties among the U.S. electorate, the post-Iraq fatigue and the post-sequestration state of the U.S. military, preventive war seems like a nonstarter. But there are other options.

^{Strengthen the Alliance}

While Seoul counts on its partnership with Washington, South Korea is no longer dependent on the United States for its preservation. Nor is it, contrary to President Donald Trump’s comments last year, free-riding on the back of the U.S. military.

Consider the division of labor: 600,000 ROK troops augmented by 28,500 U.S. troops. Consider Seoul’s increases in defense spending. Consider the ROK’s recently-unveiled “kill chain” doctrine designed to deter and, if necessary, destroy the North Korean People’s Army.

All of this underscores something the North Koreans, Chinese and even some Americans seem to overlook: The ROK is not some appendage of Washington. Rather, it’s a sovereign nation in the crosshairs of a terror state. Moreover, it’s a U.S. treaty ally that has fought and bled alongside American troops. As such, it deserves Washington’s support—not threats to tear up trade treaties or a bill for our shared commitment to peace and prosperity in Northeast Asia.

Trump’s decision in September to allow Seoul to purchase more advanced weaponry from the U.S.—including missiles with longer ranges and heavier payloads—is a hopeful sign. If the North Koreans want an arms race, the U.S. and ROK should give them one—and remind them that the West has waged and won such struggles with far more formidable foes than the Kim regime.

^{Strengthen the Shield}

Pentagon officials convinced Seoul in 2016 to allow deployment of a THAAD anti-missile system. Although ROK President Moon Jae-in delayed deployment of THAAD in June, he reversed course in August and approved four additional THAAD launchers. This adds yet another layer of protection to the South, which already fields Patriot batteries and Aegis warships. Also in 2016, South Korea joined the U.S. and Japan for the trio’s first-ever joint missile-defense exercises.

To shield Americans, Japanese and South Koreans from North Korean miscalculation or provocation, the allies need to train together more often and deploy more defensive equipment—more THAAD systems, more Aegis warships, more Aegis Ashore systems, more Patriot batteries, more missile-tracking radars in the region and more ground-based interceptors in the U.S.

^{Rebuild the Arsenal}

Of course, fielding more missile-defense assets presupposes more defense spending. Japan and South Korea have been increasing defense spending. But the U.S. defense budget has fallen from 4.6 percent of GDP in 2009 to 3.1 percent today. This is the best way to invite the worst of possibilities—what Churchill called “temptations to a trial of strength.” And here we are.

U.S. policymakers should reverse this downward spiral, restore defense spending to the post-Cold War average of 4 percent of GDP and recognize that a well-equipped military is not a liability to cut but an asset to nurture. To his credit, Trump has proposed a $639-billion defense budget for 2018—well above sequestration’s draconian limits. Congressional leaders have proposed even larger defense budgets, some as high as $705 billion.

^{Raise the Stakes}

Kim Jong Un’s generals know that a second Korean War will not end in stalemate—it will end North Korea. But does Kim know that? To make sure he understands the stakes, Washington could publicly (after the requisite discussions with Seoul) announce that the U.S. is redeploying deterrent nuclear assets to South Korea. In a sign of goodwill and confidence-building, Washington withdrew U.S. nuclear weapons from the South in 1991. Pyongyang and Seoul then agreed to keep the Korean Peninsula free of nuclear weapons. Pyongyang violated that agreement, just as it violated the Nuclear Nonproliferation Treaty, the Agreed Framework, and scores of UN Security Council resolutions related to missile tests and nuclear development.

Deploying a nuclear deterrent in the ROK is not a kneejerk reaction of fanatic-fringe warmongers. In fact, in 2010 and again this year, high-ranking ROK officials raised the prospect of redeploying U.S. nuclear assets to deter the North and provide increased force protection for U.S.-ROK troops. Nearly 70 percent of South Koreans support the return of U.S. nuclear weapons to the ROK.

In addition to noting that “redeployment of tactical nuclear weapons is an alternative worth a full review,” South Korean Defense Minister Song Young-moo recently confirmed that he asked Defense Secretary James Mattis to rotate or base “strategic assets”—aircraft carriers, submarines, long-range bombers—in South Korea.

“We need these strategic or tactical assets…closer to North Korea’s nuclear and missile sites,” Chun Yung-woo, former ROK national security adviser, said in a Washington Post interview.

^{Play the Ace Card}

Of course, Seoul could always go nuclear on its own. If Beijing continues to allow North Korea to play these deadly games, Washington could play the Seoul-Tokyo card.

Nearly 60 percent of South Koreans support developing their own nuclear deterrent. “We can’t borrow an umbrella from a neighbor every time it rains,” says Won Yoo-cheol, a leading lawmaker in the National Assembly. “Only nuclear weapons could be an effective deterrence against nuclear weapons.”

Related, Japanese government officials and analysts have, at times, hinted at Japan’s capacity to develop a nuclear deterrent—and even the need to do so.

Yes, the PRC’s media mouthpiece warns, “If North Korea launches missiles that threaten U.S. soil first and the U.S. retaliates, China will stay neutral,” and Beijing has cut coal shipments from and fuel shipments to the North. But Beijing claims it can’t fully control Pyongyang because North Korea is a sovereign nation. Washington should privately remind Beijing that South Korea and Japan are as well, and suggest that it may no longer be possible for the U.S. to keep Tokyo and Seoul from joining the nuclear club. This may be the best way to pressure Beijing to rein in its Frankenstein monster in Pyongyang. Mattis would be the ideal delivery vehicle for such a message.

^{Worst Case}

To be sure, these options—building up the military, redeploying tactical nukes, engaging in hard-nosed nuclear deterrence, signaling China’s leaders and Kim’s generals, strengthening missile-defense capabilities, repositioning strategic assets—present their own risks and costs. But they are offered as alternatives to something far worse: waging preventive war against the North or waiting for the North to miscalculate us into a war.