Vladimir Putin's surprise offer to cooperate with the United States on a missile defense capable of detecting and intercepting missiles from Iran or elsewhere has been greeted as a positive sign for US/Russian relations, and it may well be. But it also raises questions about the potential unintended consequences of missile defense.
When President Reagan first committed billions to what was then known as the Strategic Defense Initiative (SDI or, pejoratively, "Star Wars"), it seemed, in principle, like a good idea. Reagan believed---correctly---that there was something immoral about relying for our defense against the threat of nuclear attack on a reciprocal threat to attack and kill millions of innocent civilians who had the bad luck to live under Soviet rule. Much better to defend ourselves with a shield that deflects or destroys the enemy's missiles without threatening civilian lives.
But the Cold War version of SDI had two basic flaws. First, the technology didn't work, even in tests in which the defenders had advance knowledge of the flight path of a single incoming missile. Second, and relatedly, it was destabilizing. Strategic planners understood that even with advances in technology, a missile shield would never be impermeable, partly because attackers could be expected to modify their missile design or simply send many more missiles (plus decoys) to overwhelm the system. But that created a payoff for a first strike. Prior to missile defense, a first strike by the US against Soviet land-based missiles was understood to be futile because it wouldn't destroy all hardened targets, but with missile defense, in a crisis in which the US feared a Soviet strike, the US might gamble that a US first strike would be sufficiently effective that the resulting decimated Soviet missiles could be handled by the missile defense. (The U.S. was not nearly as vulnerable to this logic because even with fewer total missiles, we had many more on nuclear subs and bombers, which could not readily be hit by a first strike.) Thus, the Soviets feared that the point of SDI was to give the US a first-strike capacity, which in turn made a confrontation between the superpowers more likely.
Fortunately, the Soviet empire fell before that nightmare scenario came to pass, and now champions of missile defense argue that whatever its limits against an adversary armed to the gills (like the old Soviet Union), it makes a great deal of sense against a potentially hostile power with a small arsenal, such as North Korea today and perhaps Iran in the not-too-distant future. A defense that is porous with respect to thousands of missiles could be impermeable with respect to a dozen or so.
This strikes me as correct but it overlooks other dimensions of the strategic equation. We know that neither Iran nor North Korea would launch an unprovoked missile attack against us or our allies because such an attack would have a clear return address, resulting in the devastation of the attacking country. Conventional wisdom (which seems clearly right on this point) says that Iran and North Korea want nukes so as to deter a conventional attack and, in the case of Iran at least, perhaps to give cover to its own conventional attacks (directly or through proxies such as Hezbollah) against its neighbors. So, one must ask (as game theorists tell us Cold Warriors far too infrequently asked about their adversaries): What will Iran and North Korea infer about our intentions if we build a missile defense against their missiles? The answer, I think, is that they will assume this means we're considering a conventional attack against them, and building a missile shield so that we can withstand a retaliatory nuclear strike. If so, the logical next move for them is to beef up their own nuclear and missile capacity.
That in itself would be terrible but it could have even worse consequences, since missiles are not the only way to deliver nukes, and some methods of delivery would not necessarily be traceable to the government of a country subject to deterrence. In other words, although the mechanism is somewhat different, development of a missile defense would be destabilizing today, just as it would have been during the Cold War. Reagan's noble dream remains a nightmare.