What the next Korean war may look like

Newsweek:

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But the grim reality is that a pre-emptive strike, against North Korean missiles or nuclear facilities—or both—could well mean war. Should the day come when President Trump believes he needs to order a pre-emptive strike against targets in North Korea to eliminate a direct threat, the U.S will not be able to take out all of the North Korean artillery front loaded near the border. “Not,” says former National Security Council staffer Victor Cha, “without using tactical nuclear weapons,” which is not something the U.S. would consider, given that Seoul is right down the road. A U.S. strike, simply put, could well trigger the second Korean War.

What would another armed conflict on the peninsula look like? During the Korean War, which lasted from 1950 to 1953, some 2.7 million Koreans died, along with 33,000 Americans and 800,000 Chinese. In any pre-emption scenario now, the U.S. would try keep the strike limited to the task at hand; at the same time Washington would signal in any way it could—probably via the North’s ally in Beijing--that it did not seek a wider war.For the past two years, the U.S. and South Korea have been practicing pre-emption exercises. In 2015, they adopted a new war plan, OPLAN 5015, which includes attacks on the North’s nuclear and missile facilities, as well as “decapitation attacks” against Kim Jong Un and the rest of the North Korean leadership.South Korea also developed its own pre-emptive attack plans, and has acquired, U.S. and Korean officials say, weapons capable of destroying some of North Korea’s weapons of mass destruction. Seoul has also built an elaborate defense system, which includes the recent delivery of the U.S. terminal high altitude area defense system, which shoots down incoming missiles in the final phase of their descent.
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U.S. war planners believe North Korean forces would to try to overrun South Korea’s defenses and get to Seoul before the U.S. and the South could respond with overwhelming force. As Cha says, “as wars go, this would be the most unforgiving battle conditions that can be imagined—an extremely high density of enemy and allied forces—over 2 million mechanized forces all converging on a total battlespace the equivalent of the distance between Washington, D.C., and Boston.’’ The United States would immediately dispatch four to six ground combat divisions of up to 20,000 troops each, 10 Air Force wings of about 20 fighters per unit and four to five aircraft carriers. In Cha’s scenario, U.S. and South Korean “soldiers would be fighting with little defense against DPRK artillery, aerial bombardments, and in an urban warfare environment polluted by 5,000 metric tons of DPRK chemical agents.”Even if that artillery barrage and push into the South gave the North the initiative, there is no question, military planners all say, who would ultimately prevail in a second Korean War. The U.S. and South Korea have far too much firepower, and if Kim Jong Un decided to go to war, that would be end of his regime, whether he knows it or not.
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That first quoted sentence doesn't make a lot of sense because the idea of preemption is that a war has already been threatened and a preemptive strike is used to make it more difficult for an adversary to successfully launch.  North Korea has made numerous threat that would justify a preemptive strike against that coutnry.

I believe that the use of large conventional bombs like the MOAB and the MOP could disrupt the North Korean plan to attack civilian targets with artillery.  The US would likely have immediate air superiority over the conflict.  The North Koreans old air force would not be effective assuming they have enough fuel to fly it.  For years they have been limited in training flights for the old aircraft because of the shortage of fuel.  That is going to get even tighter if China keeps up its embargo on fuel imposed recently.  What fuel they have hoarded will likely be targeted in the initial phase to slow any mechanized operations.