North Korea now has nuclear weapons, although their current nukes are crude and unreliable. Nuclear bombs are not all that unique a weapon for North Korea. They have had chemical and biological weapons up there for decades. These, like nukes, are weapons of terror, meant mainly to intimidate without having to use them. The North Korea leadership (military and political) has been told by the U.S. that if nukes or chemical weapons are used against South Korea the northern leaders would be declared war criminals and hunted down and punished (if the hunting process didn’t kill them first). If North Korea attacked and lost, China could probably protect those leaders who fled to China. But if North Korea used chemical, nuclear, or biological weapons against the south, China would have a much more difficult time protecting those involved in carrying out that atrocity.
The North Korean army is still capable of moving across the border (the heavily mined and defended DMZ or Demilitarized Zone) but not very far. South Korea now has plans to push back once they have halted the North Korean advance and pounded it with artillery and air strikes. South Korean war plans now contain a lot of detail on how South Korean forces would occupy and take control of North Korea. This would include rounding up any military and political leaders who had not been killed or fled (most likely to China).
Although North Korea has, on paper, 1.1 million troops (equipped with over 5,000 armored vehicles, 600 combat aircraft, and hundreds of ballistic missiles and rocket launchers), this force has been falling apart since the late 1990s. To put that in perspective, South Korea (with 680,000 troops and more tanks, aircraft, and warships than the north) and spends over forty times as much, each year, on equipping, maintaining and training each of its troops. North Korean troops now spend a lot of their time growing their own food, working in factories, or laboring on public works projects. There is little money for fuel to operate trucks or armored vehicles, and even less for spare parts if these elderly vehicles break down. In the north, aircraft and ships rarely operate, which means the crews are poorly trained.
American and South Korean military planners believe that, if North Korea were to declare war (as they have been threatening to do for over half a century), the main threat would be the bombardment of Seoul, the capital and largest city of South Korea. Some North Korean artillery can reach Seoul, as can nearly all the rockets and missiles. Damage could be in the tens of billions of dollars and the casualties in the tens of thousands (or more, if chemical weapons are used). But because of the shortages, and lack of training, the North Korean troops would be unable to advance far into South Korea. And the South Koreans also have plans for using their better trained and equipped forces to try and halt the bombardment and advance into North Korea as well. For many years the advance into North Korea was thought to be a difficult option, mainly because of the large number of special operations troops the North Koreans had. But the great meltdown up north has done serious damage to this capability as well. South Korea has also been increasing ability to quickly launch air and artillery attacks against North Korean artillery and missiles aimed at Seoul. U.S. forces would participate in this as well, in addition to providing lots of intelligence data on where those weapons are.
North Korea has long maintained elite commando forces, troops who were carefully selected, then paid, housed and fed better, and given access to better equipment. About 15 percent of all troops are in these elite units. Most of them are similar to U.S. rangers, marines, paratroopers, or special reconnaissance troops (U.S. Marine Force Recon and army LURPS).
At the apex of North Korean Special Forces there are about five thousand commando and U.S. Special Forces type troops. These are meant to get into South Korea and go after key targets and people. Again, the North Koreans have trained for half a century to do this but have not been able to actually put these troops to the test much. There have been hundreds of small operations in the south over the last half century. In the 1960s there was a low level war going on, as the North Koreans sent dozens of small teams south each year. Over a hundred American troops were killed or wounded, and many more South Korean soldiers and police. Yet, the North Koreans had little ultimate success.
While the top special operations units are still well cared for, more and more reports come out of the north about many less skilled special operations troops complaining about less, or at least lower quality, food and other benefits (like access to electricity year round, and heat during the Winter.) More of these troops are deserting and heading for China, where they can be more easily interviewed. Some have made it all the way to South Korea, where the extent of their numbers and preparations has pushed South Korean commanders to increase their own security preparations and train more troops to deal with all these commandos in wartime.
While the North Korean special operations troops are grumbling, and not getting all the training resources (ammo and fuel) they need, they remain a highly motivated, and generally loyal, force. The government uses these troops to insure the loyalty of the other 85 percent of the military, and more and more elite troops are being used to assist the secret police in going after dissidents and corrupt officials. This is probably hurting the North Korean special operations forces more than anything else. The troops are getting a close look at the corruption and contradictions in North Korea. The troops generally lived in closed bases and don't get out much. But now that they do, they see a North Korea that is unpleasant and not as well as their commanders told them it was. It turns out those letters they were getting from home were not exaggerating how bad things were. And the trend has been down for so long, it's hard to assure the troops that there's any way up.