Here's a rough breakdown of the firepower on both sides of the Demilitarized Zone:
With a standing army of over a million, North Korea's ground forces outnumber that of South Korea and the United States (which has some 28,500 troops in South Korea). Of major concern is that artillery from North Korea could destroy Seoul even without the north moving any troops. "The ground forces are by far the largest and most formidable of North Korea's military forces," says a 1991 report by the Defense Intelligence Agency. "In the 1980s, Army force structure became increasingly mobile and mechanized, with a steady increase in tanks, self-propelled artillery, armored personnel carriers and trucks." Almost 20 years later, North Korea still maintains one of the largest standing armies in the world.
The U.S. enjoys undeniable naval superiority over North Korea, whose navy stays close to shore. "Most North Korean combat vessels, such as light destroyers, patrol ships, guided missile boats, torpedo boats and fire support boats, are small," says Globalsecurity.org's review of the North Korean navy. However, it added, the North's antiship missiles pose a large threat. South Korea has also been bulking up its navy with more warships, like the U.S.-build Aegis destroyer.
The United States and South Korea have a major advantage here. "The U.S. normally has about 300 fixed-wing combat aircraft based in the immediate vicinity of the Korean Peninsula," according to the International Institute for Strategic Studies. North Korea, by contrast, has a squadron of Russian fighters. More significant than the number of aircraft is the quality of the pilots; it's estimated that North Korean pilots get just a fraction of the training and flight time compared to the United States and South Korea.
No one knows precisely how many nuclear weapons North Korea has, but it has produced enough plutonium for about a dozen bombs, according to the Arms Control Association. The U.S. withdrew tactical nuclear weapons from the Korean peninsula in 1991, but it still maintains a nuclear umbrella that covers South Korea. Though the United States won't publicly discuss the details of what that umbrella includes, at least one report says it involves the possibility of a retaliatory nuclear strike against North Korean nuclear facilities.