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 $1billion to fight and cause $1trillion worth of damage.
Former Whitehouse adviser Victor Cha described how a North Korean invasion would play out in his book, The Impossible State.
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.
Any forces that do attempt to cross the Tsushima Strait into the South face waters infested with Kim's submarines, all of them told to torpedo American supply ships.
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.
It would take South Korea and its American allies several days to neutralise the enemy artillery, his book reveals, leaving hundreds of thousands dead.
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.
The obsolescence of DPRK equipment and training does not mean they are rendered harmless.
"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."