By all accounts North Korea has 3-5 Hiroshima-yield fission bombs. If recently reprocessed plutonium has been weaponized (as the North Koreans claim) they may have as many as 10. North Korea also has missles quite capable of hitting any target in Japan or South Korea with a plutonium bomb (although maybe not with a heavier uranium bomb). They also have 13,000 deeply entrenched artillery tubes within striking distance of the South Korean capital of Seoul, and have had 50 years to dig them in. They have threatened to obliterate Seoul with this artillery should a new conflict erupt, and have the capability to do it in short order. There are no good ways to prevent this by conventional means. They have also threatened to strike Japan and US forces with nuclear weapons in the event of war. This deterrence is credible, and the conventional threat alone deterred the Clinton administration from reacting to the North Korean nuclear program in the mid 90's, especially after Jimmy Carter crafted a deal that (literally) papered over the crisis.
As we now know, the North Koreans honored that deal in the breach, refraining from producing plutonium bombs, but ramping up a parallel program to make (less-optimal) uranium bombs, of which they now have several. When the US called them on it, the North Koreans just used this new nuclear shield to begin production of plutonium bombs. There was little the US could do, short of a disastrous war which would kill a million South Koreans and destroy South Korea's capital. North Korea would be obliterated in the process, but that isn't satisfactory either. Hydrogen bombs don't lend themselves to tactical strikes, kill indiscriminately, and have effects far beyond the intended targets.
Primarily in response to this situation (and other potential situations) the Bush administration reviewed the US war-fighting strategy and arsenal and determined that a new class of weapons was desperately needed: low-yield (possbily subkiloton) ground-penetrating fission weapons. Congress agreed, and the US is now researching and possbily developing such devices.
Kerry opposes them utterly.
"As president, I will stop this administration's program to develop a whole new generation of bunker-busting nuclear bombs," Kerry told a crowd of supporters in West Palm Beach, Fla. "This is a weapon we don't need. And it undermines our credibility in persuading other nations. What kind of message does it send when we're asking other countries not to develop nuclear weapons but developing new ones ourselves?"Should war come with North Korea, say after a North Korean nuke is sold to al Qaeda and blows up in New York Harbor, will the US be able to respond, defending both itself and Seoul, without widespread civilian losses? If Bush prevails, the US might have two alternatives, under Kerry only one.