IT WAS March 2019. America and South Korea were conducting an annual large-scale military exercise, Foal Eagle, involving nearly 20,000 American troops and about 300,000 Korean counterparts. The drill was taking place against a backdrop of continuing missile tests by the North Koreans.

Over the previous two years, the regime of Kim Jong Un had successfully test-launched several intercontinental ballistic missiles (ICBMs). The most recent was another two-stage rocket that analysts reckoned could reach any city in America. It carried what appeared to be a credible re-entry vehicle to shield its nuclear warhead as it plummeted through the Earth’s atmosphere towards its target and from which decoys could be fired to bamboozle missile defences.

A seventh nuclear test in January had confirmed that the warhead carried by the missile would in time be a thermonuclear one with the power to destroy all life within an area of 70 square kilometres. Some intelligence reports had suggested that the next test the North Koreans would conduct, likely to coincide with the climax of Foal Eagle, might include a high-altitude nuclear explosion of the kind that America and the Soviet Union had conducted until 1962 to test their weapons.

Whereas other new nuclear states, such as Pakistan and India, had been content to carry out all their testing underground, Mr Kim had boasted since the start of the year that he would soon provide final proof that would convince the world to respect North Korea’s nuclear capability. If the North really carried out such a test, the electromagnetic pulse it would cause could take out satellites and damage power stations on the ground.

For Donald Trump, that would cross a red line. The moment was now or never. Responding to claims by Mr Kim at the outset of what was to become his troubled presidency that North Korea was in the final stages of developing an ICBM, he had declared on Twitter, “It won’t happen!” In June 2017, a few days before North Korea conducted its first ICBM test, Mr Trump had stated that the “era of strategic patience” with Mr Kim was over.

Since then, America had tightened sanctions against the North Korean regime, including taking action against Chinese and Russian firms trading with it and cutting off sources of finance generated by Mr Kim’s criminal networks overseas. But with China only willing to offer token help, it had proved too little and too late to slow the North’s rapid development of nuclear missiles.

The president had thus far heeded the warnings of his defence secretary, Jim Mattis, and his national security adviser, H.R. McMaster. The risks of taking military action were too great, they had argued. But Mr Trump was no longer willing to listen. Mr Mattis was said to be on the brink of resignation, partly because he did not believe that Mr Kim was about to carry out an atmospheric test. Mr McMaster had been fired and replaced by John Bolton, a hawkish former ambassador to the UN who had been a prime mover for action to overthrow Saddam Hussein because of his supposed arsenal of weapons of mass destruction. Mr Bolton had told the president that a high-altitude test was imminent. He had long argued for doing whatever it would take to bring about regime change in North Korea.

Mr Trump did not necessarily want to go that far. China’s leader, Xi Jinping, had warned him that there would be “serious consequences” if such a step was being considered. It was not clear whether China would step in to help North Korea as it had in the past (the view in Washington was that it would not). What was not in doubt was its hostility to anything that might bring American forces north of the 38th parallel and close to China’s border.

South Korea’s president, Moon Jae-in, had at first been strongly against any pre-emptive strike, as his country would bear the brunt of any subsequent miscalculation by either side. But after bullying from Washington, he had reluctantly withdrawn his opposition. Mr Trump felt very strongly that he needed to show his supporters at home that he could still make tough decisions.

To that end, he had asked his military advisers to come up with a plan that would show he meant business; powerful enough to make Mr Kim think twice before hitting back, but not so drastic as to trigger war. After all, Mr Kim would surely realise that to do so would risk entering a cycle of escalation that would lead inexorably to his defeat and the obliteration of his dynasty—the very thing his nuclear programme had been designed to prevent.

The preferred option would have been to have shot down the missile in its boost or ascent phase with interceptors fired from US Navy destroyers. But new SM3-Block2A interceptors, which might have been fast enough to do the job, were not yet ready for deployment. The plan the Pentagon had therefore come up with was to fire a salvo of cruise missiles from a submarine in the Sea of Japan, destroying the missile on the ground. Much would depend on getting prompt intelligence from surveillance satellites and high-flying drones to hit the launch site before the missile could be fired. Any subsequent tests, Mr Kim would be told, would get the same treatment. A belligerent response by Mr Kim would be met by an attack on his nuclear and missile facilities.

Mr Trump was warned, however, that although Mr Kim was thought to be rational, he faced political problems of his own and would have to react in some way. According to the most optimistic scenario, Mr Kim might feel that he could get away with a gesture, such as firing missiles at the outer islands, in an operation similar to the shelling of Yeonpyeong in 2010. He might also conclude that he could anyway stop testing for now, as he had credibly acquired the means to hit the continental United States.

The operation appeared at first to succeed. There was little unintended damage and not much indication whether the missile had been armed (nuclear weapons are designed to resist accidental explosion—the warhead is encased in a sturdy re-entry vehicle and detonation sequences have to be minutely timed). Despite the usual threats from Mr Kim to “wipe America off the face of the Earth” and to turn Seoul into a “sea of fire”, nothing appeared to happen. Mr Trump’s poll ratings spiked and he tweeted: “Fat Kim just got what he’s been asking for. SAD!” Triumphantly, he berated “my generals” for their caution which had stopped him from doing something similar sooner.

But even as Mr Trump was bragging about the success of the strike, Mr Kim was ordering elite units from his 180,000-strong special operations force to carry out a series of hit-and-run attacks on targets in the South. Some would infiltrate by using a network of tunnels running beneath the demilitarised zone (DMZ); others would be inserted from the sea by mini-submarines or flown in by ancient hedge-hopping An-2 biplanes that were hard for modern radars to spot. Meanwhile, North Korea’s navy had also begun laying mines in both the West and East seas in an effort to disrupt trade. A series of cyber-attacks on South Korea’s critical infrastructure also appeared to be under way.

Holding their nerve gas

North Korea’s aim was to stop short of actions, such as releasing nerve gas in the outskirts of Seoul, that would prompt an all-out response from the Americans and their ally, but to do just enough to generate a sense of panic and uncertainty among South Korean civilians. Mr Kim’s advisers had told him that their analysis, based on their contacts in the South, was that this would generate huge pressure on the South Korean government to veto any further escalation that might lead to outright war.

That turned out to be a grave miscalculation. The allies could not be sure what Mr Kim planned next, so they had to prepare for the possibility that sabotage attacks were a prelude to a major offensive. With that in mind, the evacuation of foreign nationals, mainly from Seoul, had begun. These included some 150,000 American civilians, over 40,000 Japanese and up to 1m Chinese citizens. The evacuation was intended to send a strong message to the North that events were developing a momentum of their own.

American and South Korean commanders had recommended to their governments that they should prepare for the worst. The military exercises already under way were intended to practice OPLAN 5015, a classified scheme drawn up in response to the growing missile threat. The drill, they advised, could rapidly be turned into reality. Whereas previous war plans had been premised on the belief that a new conflict would be fought along similar lines to the first Korean war (with large units first deployed in defensive formations before counter-attacking into the North), the new plan called for precision strikes and special forces acting behind enemy lines.

The first requirement would be to suppress North Korea’s surprisingly lethal integrated air-defence system, which fields, along with Soviet-era surface-to-air missiles, the indigenously produced and highly capable KN-06. With that out of the way, missiles, smart bombs and huge “bunker busters” would rain down on nuclear sites, missile launchers and command posts while South Korean special forces carried out “decapitation” raids to kill North Korea’s leaders. The idea was that by striking pre-emptively, any war would be both limited and short.

The problem was that the commanders could only be moderately sure that their plan would work. Apart from the effort required to disable Mr Kim’s air defences, an almost complete dearth of reliable human intelligence meant that there might be secret nuclear sites that were not on the target list. Add to that North Korea’s extraordinarily mountainous terrain and its tunnelling skills, honed over the past 60 years, and there was a good chance that some nuclear facilities would remain intact. In addition, missiles on mobile launchers could be hidden deep in caves.

An air of superiority

Therefore, rather than press ahead with OPLAN 5015 immediately, American commanders decided that they should bring at least another 500 tactical aircraft into the theatre, both from carriers and from bases in America. They would be needed to maintain the sortie rate required to destroy North Korea’s air defences and then hit all the other targets, including both the ones that were already identified and also others that would emerge. Although it might take a few weeks, it would signal to North Korea the seriousness of their intent and might persuade Mr Kim not to press ahead with a wider attack.

Mr Kim was aware that time was against him. At this stage, he too hoped to avoid an all-out war, which beneath his usual bombast he knew he might lose. But the build-up of forces in the South, especially the rapidly increasing airpower that would soon allow his adversary to launch a pre-emptive attack against his most important weapons, convinced him that he had to fire a powerful warning shot of his own.

With over 14,000 artillery pieces, about 1,000 of them positioned in caves and bunkers within range of Seoul, he could do a lot of damage quickly. But unleashing the kind of barrage that his regime had threatened in the past would take him rapidly past the point of no return. He also had to decide how much of his long-range artillery force of 170mm guns and both 240mm and 300mm multiple-rocket launchers he was prepared to expose at this stage to counter-battery fire from the South. He therefore opted for a limited salvo that would last under an hour before pulling back his artillery to positions where it would be less vulnerable.

His message to Mr Trump was that this was just a taste of what South Korea and its allies could expect if he continued with his aggressive war plans. It failed to have the effect that Mr Kim was hoping for. Despite hints that he might stop there, with several thousand civilian and military casualties already sustained, American and South Korean commanders had to take action in case this was just the prelude to an all-out artillery barrage.

Based on attempts to model the effects of such an attack, they believed that in just a few hours up to 100,000 people would be killed in Seoul and perhaps many more if they did not act fast. That meant putting OPLAN 5015 into action immediately and with it a warning directly from Mr Trump to Mr Kim that, if he launched a missile believed to be carrying a nuclear warhead, he could expect a swift and devastating nuclear response that would “remove him and his country from the map.”

The ferocity of the initial assault stunned Mr Kim. Large parts of his massive but technologically crude military infrastructure started disappearing. Tank divisions he had ordered south were sitting ducks in the narrow valleys they were forced to pass through. Any artillery that had been left in the open was being systematically destroyed by witheringly accurate counter-battery fire. Missile launchers supposedly hidden in caves were being pulverised by huge bunker-busting bombs. Twice Mr Kim had narrowly avoided being blown apart himself, when bombs had hit command bunkers minutes after his departure.

Faced with the imminent destruction of his regime, Mr Kim decided to go down fighting. The artillery he had held back began its bombardment of Seoul. A number of the shells and rockets had chemical warheads. Special forces already in the South were ordered to release poison gas in populated areas. Rumours rapidly spread of the use of biological weapons.

Most fatefully, Mr Kim, realising that his time would soon be up, had made up his mind to launch what remained of his nuclear arsenal. He cared little about the consequences either for his enemies or his own long-suffering people. He lived just long enough to know that neither of his two ICBMs had left its launch pad and three Musudan intermediate-range missiles, aimed at Tokyo and the American base at Okinawa, had been shot down by Patriot batteries in Japan before they could reach their targets. The new THAAD system and Patriot interceptors in South Korea had taken care of several medium-range Pukguksong-2 missiles. But to his satisfaction, two short-range missiles, hidden like needles in haystacks among multiple salvoes of conventionally armed rockets, had got through to Seoul.

The price of defeat

The initial death toll was put at 300,000, but the effects of radiation would mean that many more would die in the months ahead, including large numbers of American civilians and service personnel. Mr Trump was advised that he had no option other than to retaliate with a nuclear strike on the North. The decision was taken to use America’s latest nuclear bomb, the guided B61-12, dropped by a B2 stealth bomber. It was both highly accurate and could have its explosive power dialled down to reduce civilian casualties and fallout. At least that was the hope.

After four had been dropped, North Korea’s war was over. Mr Kim and most of his high command had been vaporised in their bunkers, his missile force and nearly all his artillery had disappeared. Despite the use of relatively low-yield weapons, military casualties were in the hundreds of thousands. Over a million people were trying to leave Pyongyang, the capital, in case of further attack. With order breaking down and food supplies getting scarce, China found itself facing a humanitarian catastrophe on its border. It claimed that lethal radioactive material was being blown into Chinese cities by disrupted weather.

Nobody knew how an appalled President Xi would respond. The shock sent stockmarkets across the world reeling, foreshadowing a global recession to come. Mr Trump, however, was undaunted. He tweeted: “Nuke attack on Seoul by evil Kim was BAD! Had no choice but to nuke him back. But thanks to my actions, America is safe again!”