Six months of conventional high intensity war by the 700,000 man South Korean Armed Forces and US Military power would utterly destroy the 1.1 million man NKPA.
Civilian casualties both North and South would be catastrophic.
Most of the North Korean air and sea power would be destroyed within the first 72 hours. Only two NKPA divisions --Special Ops and Guards --are well equipped and effective. These troops are primarily focused on regime security from internal threat.
Much of the NKPA ground forces are malnourished and badly trained conscripts. Equipped with old technology.
Even the much vaunted NKPA conventional artillery force within range of Seoul would likely be largely destroyed by US Air, and killer UAV's, and South Korean counter-battery artillery fire within a week.
NUKES are the ultimate problem. The only reliable way to assure their destruction is by US nuclear pre-emptive strike. Probably take 70 plus nuke weapons. The US will never execute this option UNLESS WE ARE SURE THE NORTH KOREANS ARE ABOUT TO LAUNCH.
There is no possibility that the North Koreans will give up their nuke program. They will have a usable ICBM capability within the decade. Possibly will possess a SLBM capacity within 15 years. Almost sure to have 100 plus nuke weapons within a decade.
The US and our regional allies need greatly enhanced anti-ballistic missile capability. South Korea. Japan. At sea. Airborne continuous killer drones. Directed energy weapons for anti-missile defense. Enhanced US strategic ballistic missile defense in HAWAII and Alaska and in California.
The US faces a serious strategic dilemma. There are no realistic options except enhanced defensive measures and the deterrent threat of a devastating US nuclear response.
GENERAL USA RET