Superpower politics and Mutually Assured Destruction are a totally different game than restraining a lunatic street punk with a couple of nukes on some questionable missiles. Any ABM system sufficient to discourage the NKs need not be sufficient to provoke another arms race or panic someone into gearing up for a nuclear first strike.

The first ABM system was installed by the Soviets as a defense of Moscow, and it sent the US president at the time (I don't recall whether it was Johnson or Nixon) to general quarters, and rightly so. It threatened the balance of power and immediately provoked our installing a spot defense system of our own. It soon became clear that a whole new expensive arms race was about to take off, and that neither side could guarantee it would be effective, but that war would be much more likely now that a first strike became a more feasible option--ABMs are more effective against retaliatory strikes than first strikes.

The excuses from the Russians were oddly similar to the ones Reagan made years later when he was promoting Star Wars ("how can anyone object to a purely defensive weapon?"). In the topsy-turvy world of MAD, there are no purely defensive weapons. It works like this: If you launch a pre-emptive attack, many of your weapons will get past your enemy's ABM, but when he retaliates with his now highly weakened force, YOUR ABM system will have a lot less to deal with. Besides, you'll have much fewer targets to defend since your silos will be empty.Consequently, not only does ABM make a first strike more attractive for either party, it makes it almost the only viable option for the weaker party. This does not lead to a stable situation in a crisis.

Hardball diplomacy, and the realization they couldn't match us on technological development, quickly convinced the Soviets to sign a treaty freezing ABM development which both sides honored until Star Wars came along.

The situation in Korea is different, the NKs will never be able to mount a strike comparable to what the Russians could, at most a few missiles. We also knew the USSR was not run by a group of madmen, N. Korea definitely is. We can certainly deploy a small, cheap system that will make the Koreans (or Iranians) think twice, but which will not be a threat to the Chinese or the Russians and start the nuclear arms race over again. Reagan's Star Wars was crazy, Bush's (and I believe even Clinton before him worked on it) is a good idea whose time has come.