What will happen if South Korea and North Korea go to war?

It could prove to be one of the deadliest and costliest conflicts in recent history. North Korea has the fourth largest Army in the world with over 1 million soldiers while its neighbor South Korea has roughly about 600,000 troops. The United States also has a contingent of 30,000 military personal stationed in South Korea and is obligated to protect South Korea in case of an attack.

There have been many reports that in the event of a North/South Korean war the United States would use low yield nuclear weapons. Another factor in play is what China's repose would be. Despite economic reforms China has been an Allie of North Korea for decades. Although the US and South Korea would eventually win China could most certainly be wary of expanded US influence along its border might very well intervene on North Korea's behalf.

Lets just hope that it does not happen! :)  It is truly a completely different situation since the Korean war where the South Koreans were massively out numbered and lacked the weapons to stop the massive push from the red army whom used their huge army literally as meat shields. The modern war is won with economical power and technological might not with human meat shields.

The former is a first world country while the latter is a 3rd world country comparable to Somalia who's main avenue is from selling obsolete soviet era weapons they developed in the basement to other 3rd world countries and narcotics.

South Korea which has the 12th largest economy in the would would be brought to its knees. Millions of Korean refugees would try to enter China, the world economy would take a huge hit and oil prices would most definitely sky rocket. There would be utter destruction and chaos on both sides of the peninsula. The Korean War had roughly 3 million deaths which is astronomical. Couple that with today's modern weapons and you could easily get a number 3-5 times that.
Although North Korea is technologically inferior to South Korea and the United States its will to fight is not. The North Korean military is very well trained and disciplined and many experts predict they would fight to death such as the Japanese did in World War II. North Korea spends a vast amount of its GDP on weapons and its military and has nearly 10,000 hidden artillery pieces pointed at Seoul (South Korea's capitol). North Korea has also been preparing for an invasion by the United States and South Korea for decades and they have built many hidden bunkers and and under ground tunnels underneath the DMZ in preparation for an attack. Not to mention that North Korea is believed to have a small contingent of Nuclear weapons which it could detonate in South Korea, Japan, or any other country it sees fit.

Secondly I truly doubt the where the morale of the current NK army stand and its actual supposed numerical threat since its army is very poorly trained, under equipped, and with the current spread of digital technology more and more people in North Korea understands the corruption that is the North Korean government its just sadly they do not have the power to fight it (I can bet you more than 50% of them will desert their positions and run to the Chinese border or the South Korean border for freedom once a serious war happens). Versus an army armed to the teeth defending their capital (imagine Washington sharing the border with Iran. scary ain't it?). Consider poor poverty stricken farmers with sticks what they call guns versus healthy, well trained and highly motivated troops.

Military tech wise South Korea is on par or even surpasses the US in several areas; mainly ballistic missile technology, Nuclear technology (South Korea HAS capabilities to produce nuclear missiles), fighter jets (the South Korean variant of the F15k is the only fighter model in the world to be more advanced than the original US fighter), submarine/Aegis class destroyer technology (one of the only reasons they do not build nuclear subs is because they are not suitable for defending the shallow Korean shores (they are too loud and nuclear subs are designed for blue water navies) and also because of American political pressure) and lastly mobile artillery technology. Currently the only things keeping the Korean military back from being truly scary is political pressure from the US (nukes, ICBMs, nuclear submarines and aircraft carriers are all not allowed!!! How annoying, how are they going to scare the Chinese and Japs away?) (One of the examples are Korea had to outsource the rockets to Russia to propel their satellites to space because US prohibited them from developing that technology fearing they might use it to develop ICBMs, truly a pain in the ass I have to say)

If a war does happen NK's nuclear arrays, artillery, radar will all be wiped in a second with South Korea's enormous barrage of ballistic missiles/counter artillery/Marines (second larges in the world next to US)/air strikes etc. NK's anti air defenses will be wiped out and South Korea's massive number of 3rd generation MBTs, IFVs, Soldiers will just blitzkrieg into the entire country in a couple of days. NK's navy? laughable with mainly small subs and patrol boats suitable for a hit and run tactic (does not work in a true all out war). NK's air force ?

 More like a joke with aging pre soviet era jet planes with barely any parts and crew training. NK's massive number of special forces (behind the lines assassination and infiltration which is very lethal since South Koreans and North Koreans look the same and speak the same language so they are very very hard to detect once they are across) and nuclear weapons are the only threats South Korea currently faces.

 (BTW South Korea's capital IS COVERED with defenses I mean one of the most secure in the world so no worries about civilians they have precise evacuation procedures and bunkers (though no longer practiced much) ) Then if by any chance the war get prolonged then just keep this in mind, most of Korea's weapons are indigenous developed meaning in wartime all weapons can be mass produced much quicker than any countries can supply plus

South Korea has one the largest ship building industry, chemical plants in the world and at a moment's notice can mass produce warships, ballistic missiles and the chemical plants that produce perfumes can be changed to mass produce biological/chemical weapons within 4 days of the start of the war. Nuclear weapons can be produced in less than 3-6 months into the war. Also if you count in the massive man power reserves it is very hard to beat.

WikiAnswer: Ganderton