Of course, all may not go well for the North; Pyongyang's close friend China may head it off at the nuclear pass; the North might experience problems building more nuclear weapons; or the U.S. could strike a deal with the North that stops its growing arsenal. In short, we can hope for the best but should be prepared for the worst.

Why then should we be concerned about North Korea's WMD arsenal now and in the future? It boils down to seven reasons.

1) The threat of a nuclear attack : Right now, a North Korean attack on the continental United States is not likely because its missiles cannot fly far enough carrying a heavy nuclear warhead. However, they may make progress in the future, and Secretary Hagel's announcement that the U.S. plans to position interceptors in Alaska and possibly on the east coast to shoot down North Korean missiles is important.

The more immediate danger, however, is that the North could use nuclear weapons against its neighbors, South Korea and Japan. If there is a war on the peninsula, experts believe Pyongyang could probably put nuclear warheads on shorter-range missiles and attack major population centers. A North Korean nuclear bomb dropped on Seoul or Tokyo would inflict enormous devastation, killing or injuring an estimated two million people in each city, not to mention radioactivity that would last for decades. Pyongyang's ability to inflict horrendous damage is only going to grow if it builds new and more powerful bombs.

2) The potential for increased WMD exports. The North has already tried to help Syria build a nuclear reactor that could produce materials for nuclear weapons. Luckily, that reactor was destroyed by Israeli warplanes in 2006. But if Pyongyang's inventory of nuclear bombs grows, its technological base expands, and its need for hard currency to help cope with international sanctions increases, the North will have a larger inventory of merchandise, plus the incentive to look for overseas buyers. It's worth noting that some exports can be done just with the push of a button -- that's all it takes to transfer a bomb design in today's connected world.

Even if we detect transfers, because Pyongyang's own nuclear security blanket may grow, the international community will be less able to stop exports. While some Americans argue that the U.S. should simply attack North Korea if it is caught sending nuclear bombs or technology to our enemies, launching military strikes against a North Korea that has a bristling arsenal of weapons and is not afraid to threaten or even use them would be extremely risky.

3) Growing strains on the U.S. nuclear umbrella provided to our allies. America's alliances with South Korea and Japan are designed to protect them from attack. That includes not just stationing U.S. troops in those countries, but also a willingness to threaten and even use nuclear weapons in their defense against other nuclear-armed countries. While some experts question how effective our "nuclear umbrella" has been, South Korea and Japan see it as vital. It stands to reason that if the threat from North Korea's nuclear arsenal grows, the stress on the U.S. umbrella will also grow, requiring constant reassurance for our allies. The U.S. sent B-2 and B-52 bombers able to carry nuclear weapons to South Korea during the current crisis to calm our ally's growing security concerns while telegraphing a warning to the North. Whether such an approach will remain effective in the future is unclear.