In a secret underground base, Command Post Tango, the combined headquarters of the U.S.-South Korean command, is abuzz with activity. North Korean artillery has pummeled sites around Seoul, leaving thousands of South Korean and American civilians and service members dead. A toxic combination of North Korean provocations and U.S. escalation has prompted the North to launch a last-ditch effort to seize the whole peninsula. As the generals fill an auditorium-sized sand-table battlefield showing the disposition of friendly forces and the extent of likely follow-up attacks, hundreds of thousands of South Koreans are displaced in and around Seoul, seeking shelter and safety. Reports of North Korean insurgent strikes are streaming in as computer screens flash with alerts of cyberattacks on Seoul’s infrastructure, taking water and power off the grid and paralyzing attempts to help the civilian population. This is a fictional scenario, but an all-too-possible one. As a U.S. Army intelligence officer in South Korea, I helped prepare for various war scenarios by testing assumptions and refining war plans in several theaterwide exercises. Put fears of full-blown nuclear war aside for a moment. We’ve never been closer to a conventional North Korean attack on South Korea, and I can attest that the U.S. military knows how devastating the consequences would be. We can expect a massive humanitarian crisis, enormous loss of life, and economic disaster. There’s almost no doubt that the North would lose — but in going down Pyongyang could take much of the Korean Peninsula with it.

This undated photo released by North Korea's official Korean Central News Agency (KCNA) on May 30, 2017 shows a test-fire of a ballistic missile at an undisclosed location in North Korea. (STR/AFP/Getty Images)

Pyongyang goes all in

What could provoke North Korea to start such a conflict? Kim Jong Un may eventually believe that he has no choice, given his country’s chronic humanitarian crisis, with an estimated 41 percent of the population undernourished, and additional sanctions threatening to bring back the famines of the 1990s. Kim could calculate that a surprise attack on South Korea would rally the population and remind the world of the North’s power. And the belligerent rhetoric of North Korean propaganda, with its talk of triumph and total war, could leave its own leaders mistakenly convinced that they would win such a conflict, just as the sloppy toughness coming out of the White House might leave them convinced that the United States is about to strike anyway.

Whatever the prompt, once the decision is made to attack, North Korea will move swiftly to accomplish its war objectives — either to seize all of its southern neighbor and make itself de facto master of the peninsula or to execute a limited attack to remind the world of its teeth. From the beginning, the North will operate on a ticking clock. The logistical capabilities of the North Korean military, assuming only limited wartime assistance from China at best, will only last for a few days before the country runs out of food, ammunition, fuel, and water. Some units may be able to operate for as long as a few weeks, but maintaining supply lines across mountainous terrain will be an almost impossible task.

The North will most likely lose its major command-and-control infrastructure in the first few hours, crippling its ability to communicate across the battlefield. U.S. air power will target major brigade and division headquarters in the hope of leaving North Korean troops cut off, in confusion, and unable to launch coordinated attacks.

That will leave the North with only a brief window to entertain dreams of victory. Like the Imperial Japanese Army in World War II, the North will seek a decisive battle that, in its view, could knock out a weak-willed United States. That means a massive barrage in the first few hours of the conflict, targeting the largest U.S. military garrisons along the Demilitarized Zone (DMZ) and in the Seoul region. Other targets could include air and naval bases in the South, and possibly Japan, to prevent an allied counterattack and soften defenses for a possible entry by the North Korean military along the DMZ or via small-scale amphibious landings in the east and west. Pyongyang will fire short-range ballistic missiles and multiple rocket launchers near simultaneously to destroy these few dozen high-value defense infrastructure targets.

Although estimates vary, some figures indicate that North Korea has approximately 1,000 missiles positioned across the country and most of them within reach of Seoul. Even a small number of missiles fired into the city, targeting South Korea’s defense complex, will have a significant impact, as each warhead weighs between 500 and 1000 lbs. To put that in context, each would be enough to annihilate anything in one to two city blocks.

The missile attacks won’t last for long, though, as South Korean and U.S. firepower will pick up their locations quickly. But the North’s artillery ability to quickly retreat into underground complexes or deep into caves after attacking will make it very difficult to destroy the weapons immediately. With decades to prepare, the North’s artificial cave networks far exceed even al Qaeda in Afghanistan or the Vietnamese before the battle of Dien Bien Phu.

Artillery on its own will wreak havoc, but it is just one part of the North’s deadly arsenal. Since the end of the Korean War, the North has developed asymmetric capabilities in areas such as biological, chemical, and cyber. Some estimates indicate that North Korea possesses 2,500 to 5,000 metric tons of chemical weapons including nerve agents like sarin and VX. Biological weapons such as small pox and anthrax may also play a role. In the brazen assassination of Kim Jong Un’s half-brother, Kim Jong Nam, the world saw that North Korea could effectively use chemical weapons.

In the event of a war, North Korea will not hesitate to launch chemical and biological weapons at South Korean and U.S. air bases or on main supply routes. A biologically or chemically contaminated site would have to be treated with special care, requiring all forces in the area to don protective gear and severely disrupting South Korean and U.S. movements across the battlefield. Delivering these payloads would not be challenging, as North Korean missiles are capable of carrying chemical and biological weapons in place of conventional explosives. Decentralized attacks could also be in the cards, as North Korea has reportedly recruited hundreds of spies across the world to conduct various missions. Those agents would likely be blended into the larger North Korean population and could be activated to carry out attacks using weapons of mass destruction in the South.

But the North’s cyberprogram could be even more frightening. In the past few years, North Korea has allegedly had a hand in various cyberattacks including the deployment of the WannaCry ransomware, theft of money from Bangladesh’s central bank, and leaks of confidential data from Sony Pictures. The secretive Bureau 121 participates in offensive cyber-operations and has establishments across the world that could ensure that there will be no interruption of cyberattacks even in the event of heavy South Korean and U.S. counteroffensives on North Korean soil.

Given the country’s past attacks against financial systems, North Korea could shut down the major Korean and U.S. banks, precluding millions of transactions and denying access to credit. The economy as a whole would grind to a halt, leaving cities in crisis as “just-in-time” delivery systems fail, businesses crumble, and stores empty in panic. Another attack could shut down the energy grid in the Seoul region. Food would turn bad, patients would struggle to survive as medical equipment failed, and personal communications would collapse. The South Korean and U.S. military and emergency services would be forced to divert resources to manage the chaos, allowing the North to push deeper into the peninsula.

Whether confined to conventional artillery or supplemented by unconventional warfare, within the first few hours of the conflict, tens of thousands of people will be dead and large swaths of Seoul in smoldering ruins. The South Korean capital is one of the most densely populated places in the world; some 43,000 people live in each square mile of the city. That’s almost four times the density of Washington, resulting in horrific scenes even from a limited strike.

The United States will quickly activate its evacuation plan to gather all U.S. civilians in Seoul and move them to nearby bases or cities to airlift them out of the war theater and to countries unaffected by the war, though many will be lost in the chaos. Millions of Seoul residents will attempt to flee their homes; however, the frequent artillery attacks, patchy gas supplies, and blocked roads will make any escape from the city challenging.

There are some other options. Seoul residents have subway stations, tunnels, and bomb shelters where they could seek cover from the artillery barrage, as they have repeatedly drilled for. Several stations can hold thousands of people and were dug deep underground with a dual-use purpose. Some even have backup generators and water facilities. The South Korean government will attempt to establish makeshift camps, away from the intense fighting, to provide basic necessities, but the feasibility of such a system is uncertain. What is certain is that it would trigger one of the greatest humanitarian crises in recent history, leaving millions of Koreans internally displaced refugees.