What would a U.S.-North Korea war look like?

With tensions running high, what would a military confrontation with Kim Jong Un's North Korea actually involve? Here's everything you need to know:

Is a limited war possible?

Probably not. Trump administration officials recently said they were working on four or five military options that would not involve the total destruction of North Korea. One such option might be an air or drone "decapitation" strike to take out Kim himself. But assassinating the Hermit Kingdom's paranoid dictator would be extremely difficult; to avoid just such a strike, he reportedly has 30 residences, each with its own underground bunker, takes elaborate precautions to hide his location, and moves around only at night. All other military scenarios would lead to a catastrophic number of casualties, according to war games staged by the Pentagon and various think tanks. Kim has one of the world's largest artillery arsenals, and would likely respond to any attack by unleashing a blistering retaliatory assault on Seoul's 10 million people, killing up to 100,000 residents in a day. The region could then spiral into an all-out, months-long war costing $1 trillion in damage and a million lives, according to one Pentagon estimate. It "would probably be the worst kind of fighting in most people's lifetimes," Defense Secretary James Mattis has said.

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How would a hot war begin?

If either Kim or the Trump administration misread each other's belligerent rhetoric or aggressive actions as a signal of an imminent attack, it could trigger the other to launch a pre-emptive strike. But many of North Korea's nuclear devices are hidden in the mountains or buried deep underground, and it would take the U.S. four or five days to destroy even Kim's conventional artillery, according to the Nautilus Institute, a think tank. The megalomaniacal tyrant would likely go into "use or lose" mode and try to inflict as much damage on his enemies as possible while he still had weapons. Even if he didn't launch some of his 20 or so nuclear warheads, Kim would still have some 8,000 conventional rocket launchers and artillery cannons dug into the mountains on his side of the demilitarized zone (DMZ). They include 240-mm and 300-mm rocket launchers that can reach Seoul, located just 25 miles from the border. Some of these shells could be loaded with nerve gas or other chemical weapons, and if they "start plopping down in the middle of the city," says Joseph Bermudez, an analyst for the U.S.-Korea Institute, "there would be panic like you would not believe."

What then?

As Seoul's residents raced to the city's 3,300 bomb shelters, the full might of the U.S.–South Korean war machine would spring into action. The U.S.–South Korean command would direct dozens of jets and Tomahawk missiles into the North to destroy military bases and Pyongyang's major command-and-control infrastructure — though military experts say it would take at least four days to achieve this objective. If the U.S. also wanted regime change, a ground war would then begin. Former National Security Council staffer Victor Cha believes the Pentagon would deploy combat divisions of up to 120,000 troops to supplement the 28,500 U.S. soldiers already in South Korea, as well as South Korea's 650,000 active-duty troops. But those combined forces would still be outnumbered by Kim's million-man army, and the ensuing battle conditions would be "unforgiving," says Cha — "over 2 million mechanized forces, all converging on a total battle space the equivalent of the distance between Washington, D.C., and Boston."

What else could Kim do?

Some 100,000 highly trained special-operations troops could infiltrate the South via underground tunnels, mini submarines, and biplanes to wreak havoc, detonating dirty bombs, assassinating government officials, and sabotaging water plants. If he began to fear for his regime's survival, Kim could start aiming his medium- to long-range missiles at Japan, the U.S. territory of Guam, and even the continental U.S. itself. His last resort would be to go nuclear, knowing it would prompt the immediate destruction of his own country in a devastating U.S. nuclear counterattack. Whatever happens, Pyongyang would ultimately lose — but not without first causing mass destruction and death.

And after the fighting ended?

If Kim's regime collapsed, a vast wave of North Korea's 25 million poverty-stricken and malnourished people would try to flee the country for food and safety. Some might head south, but many would cross into neighboring China. This is the scenario that Beijing most dreads, and China might send its military to create a 50-mile-wide buffer zone to hold desperate North Korean citizens. The United Nations' Blue Helmet peacekeeping forces would likely intervene to restore some stability, amid general panic and anarchy. "It could be the mother of all humanitarian relief operations," says former Army Special Forces Col. David Maxwell. Meanwhile, U.S. forces would race to gain control over the regime's nuclear, biological, and chemical weapons, and try to prevent another dictatorial regime from rising. A 2013 Rand Corp. study warned that U.S. troops crossing into the North might trigger a military confrontation with China. "If we intervene and the Chinese run into our people and if we run into their people, what are we going to do?" asks study author Bruce Bennett. "All of that needs to be really thought about seriously."

South Korea's 'decapitation' unit

South Korea recently unveiled its own "nuclear" weapon against its dreaded neighbor: an elite "decapitation unit" whose aim is to infiltrate the North and take down Kim. In 1971, a similar kill squad was tasked with slitting the throat of Kim Il Sung, Jong Un's grandfather. Formed of thugs and street criminals, the bandit group rebelled over how it was treated, got into a firefight with South Korea's own soldiers, and was disbanded. The new decapitation unit is more professional, but faces long odds against success. First, Kim's assassins would have to make it into Pyongyang without detection, and then track down North Korea's elusive leader. Finally, they'd have to overcome Kim's elite guards. All in all, says Leonid Petrov, a Korea expert from the Australian National University, "Kim is more likely to die of an overdose of expensive Cognac or cheese [than] from a South Korean bullet."