Reports from the country reveal Kim Jong-un has built a replica of the South Korean presidential palace to train his troops for an invasion as tensions escalate.
And with America pledged to defend the South in case of attack, the West faces a war that will cost $100billion to fight and cause $1trillion worth of damage.
Special forces would invade first in a series of predawn airdrops and shore landings, sabotaging power stations, communication networks and bridges in order to “paralyse the population”.
Then “the largest artillery force in the world” would pound the South Korean capital Seoul with shells at a rate of 500,000 per hour — leaving its people only 45 seconds to take cover.
An arsenal of 600 chemically-armed missiles would cripple airports, making escape impossible, while 100 more trained on Japan would slow the arrival of US reinforcements.
In the meantime 700,000 North Korean troops and 2,000 tanks would pour across the border, with invasion tunnels discovered as deep as 475ft down — some capable of shifting 30,000 fighters an hour.
With millions fleeing, the road networks would be impassable, leaving defensive forces helpless as the enemy races across the 50 miles between the border and the capital.
And the battlefield would be polluted with up to 5,000 metric tons of chemical agents, including nerve gas, mustard gas, choking and vomiting agents, and perhaps even weaponised diseases.
“As wars go,” writes Mr Cha, “this would be the most unforgiving battle conditions that could be imagined.”
He writes: “Short of dropping tactical nuclear weapons on the battlefield it would be impossible to neutralise all of this without the North first inflicting major damage on Seoul.”
Ultimately superior technology means the US and South Korea would win, “but not without four to six months of high-intensity combat and many dead,” says Mr Cha.
“No matter how old the gun or artillery system is, it can still fire on Seoul and do damage — [but] it does mean they will ultimately be defeated by US-ROK combined forces.”
And Kim’s defences mean invading the North is a fool’s errand, with the Pentagon predicting hundreds of thousands dead as the best case scenario.
Fearing an invasion, the repressive regime has left only one paved road between Pyongyang and the border — easily targeted by the North’s bombers.
Kim’s dad kept 1,000 of the very best as his personal guard, no two of them related, in order to ensure their first loyalty was to the leader.
Command centres, factories and food and fuel depots with six months of supplies are also buried beyond the reach of American bombers.
While beneath Kim Il-sung square, in the heart of Pyongyang, a bunker command post with fresh water, ventilation and a secret escape tunnel is believed to have room for 100,000 men.
Mr Cha writes: “After the experience of American bombing and napalm during the Korean War, the North sought refuge by burrowing deeper underground than anyone else in the world, making the country like a block of Swiss cheese, with caverns and tunnels everywhere.”
So far the sheer cost to both sides has kept the peace, says Mr Cha, but with the regime becoming increasingly unpredictable, it’s unclear how long the balance will last.
The ex-Whitehouse adviser continued: “There are other dynamics, short of an all-out war on the Korean Peninsula, that are equally dangerous.
“The danger today is that one of these variables could lead us to war in Korea in a manner very different from that in June 1950.”