The more credible scenarios are that North Korea could inflict up to 1 million casualties on South Korea and possibly land a few missiles into Japan if the North Korea went into a inflict maximum damage mode. South Korea and the USA would be able to beat down and contain North Korea. The situation would then transition into a prolonged Iraq-Afghanistan. North Korea also has thousands of tons of chemical weapons.
The US should be able to strike and rapidly cripple any medium and long range missile strike launch sites.
The US could use stealth fighters and stealth bombers to degrade North Korean missile capabilities and nuclear weapons within a few days. The US would then be able to dismantle the North Korean air force, air defenses and other missiles over the next two weeks.
South Korea and the US would be able to take out any regular tanks and artillery within about a month.
An uncertainty is if North Korea would be successful in embedding 100,000 commandos into Seoul for a more protracted urban warfare situation.
North Korea has a lot of tunnels and would have a large military force that would likely be under instructions to use IEDs and other guerilla tactics to make a long term mess of North Korea. There would also be a refugee and starvation situation in the aftermath.
North Korea has more missiles than the THAAD system could shoot down in a full scale attack. However, the system would be useful if satellites could detect where more dangerous missiles might be launched. North Korea might have a dozen nuclear small nuclear weapons. North has a lot more chemical weapons.
The South Korean Ministry of National Defense estimated in 2012 that North Korea had a stockpile of between 2,500 and 5,000 metric tons of chemical weapons. Annual production is estimated at 4,500 tons in peacetime and 12,000 tons in wartime.
North Korea is believed to have highly lethal nerve agents, which work by disrupting the human body’s nervous system, resulting in asphyxiation. North Korea is believed to have stockpiles of sarin, soman, tabun, VM and VX nerve agents.
According to longtime analyst Joseph Bermudez, North Korea is believed to have specialized in “sulfur mustard, chlorine, phosgene, sarin and the V-agents.”
The Korean peninsula is relatively short; from Hyesan on the North Korean/Chinese border to the southern tip of South Korea is less than 500 miles, or the distance from Portland, Maine to Baltimore, Maryland. Pyongyang to the DMZ is only 100 hundred miles and only 120 miles to Seoul.
As of 2014, the U.S. Department of Defense estimates North Korea has less than one hundred short-range missile launchers of all types, including the Toksa/KN-02 Viper (a derivative of the Russian SS-21 Scarab) with a range of 75 miles, and its collection of Scud missiles, with a maximum range of 185 to 625 miles. Toksas and Scuds would have to be based close to the border.
North Korea also has less than fifty launchers for its No Dong missiles. Developed using Scud technology, No Dong has a range of 800 miles, making it useful for striking from deep inside North Korea against South Korea and Japan.
North Korea is believed to have 5,100 multiple rocket launchers and 4,400 self-propelled artillery pieces. Rocket artillery of 122-millimeter or greater and field artillery of 152-millimeter or greater would be capable of firing chemical shells. The majority of Pyongyang’s artillery would be capable of delivering chemical strikes.
The North Korean People’s Air Force is capable of delivering chemical weapons by air, but its aging fleet of airplanes are less reliable and less likely to get through South Korean defenses than other means.