Actually, no it’s not.
Well, let me step back for a minute and qualify that answer. Some fractional effectiveness of defense (since the arbitrary figure of 90% has been tossed out, we’ll use that) does provide for the ability to make a response strike against an attempt at a disarming first strike. If your goal is to maintain a certain percentage of your arsenal against such an effort, then an ABM system provides for an effective strategy. The mentality here is to avoid any stategic confrontation by making the risk to the opponent to great for them to consider launching such a strike.
However, their response to such a system, whatever it’s effectiveness, is going to be to double down their bets; instead of sending 300 missiles, they’ll send 600. Increase your ABM arsenal by a factor of two, they’ll increase the LVs by a factor of four, et cetera ad nausum, until even a 10% leagage is sufficient, if not to prevent a counterstrike, then to totally vanquish their opponent. The resulting arms race is both extremely costly and results in a game that is worse in every result. This was the conclusion that Secretary of Defense Robert McNamara and his theorists came to, and was his reason for opposing the instantiation of of Nike-X/Sentinel. His argument was that no cost was to great to bear in defense against nuclear attack, but that the Nike-X ABM system would make the damages (measured “megadeaths” in the curious and detached jargon of strategic assement of nuclear Armegeddon), and thus was not worth any price. Eventually, he relented to political pressure to agree to some kind of a limited system, and nearly a decade later, under the Nixon Administration, Safeguard was deployed at one site (the Mickelson Ballistic Missile Complex near Grand Forks, ND) as a point defense system and then quickly shut down.
Although the reasons for the shutdown were largely political, the system itself was obsolete by the time it was deployed. By 1975 our launch discernment and tracking systems were sufficiently advanced that a disarming first strike was effectively impossible. By the time any enemy RVs could come sailing into the complex, our birds would already have flown; indeed, while in the days of the liquid fueled Atlas a signficant delay existed between detection of a launch and getting an Atlas prepared for launch, by 1975 the fleet largely consisted of solid fuel Minuteman II and III boosters which could launch within seconds of an order, each carrying a payload of multiple RVs. Trying to destroy the silo would be like figuratively trying to close the barn door after the cows had not only left the barnyard but were in fact running through the neighbor’s pasture. Similarly, our ballistic missile submarines (then) provided a nearly unstoppable threat, further oblivating the effectivness of striking at land-based missiles.
As for the engineering problem of trying to physically intercept an incoming RV with another missile, it is certainly difficult though not impossible. The short-range Sprint missile, while nuclear tipped, was actually accurate enough to act as a hit-to-kill vehicle. The Sprint was, of course, defending an established point, so any incoming booster would be on a converging path, whereas an attempt to hit an incoming RV at a higher altitude would have a larger envelope. However, just by overwhelming the system, i.e. sending in more RVs than there were interceptors to reliably intercept them, the system could be breached. With large MIRV-based boosters like the American Peacekeeper and the Soviet SS-24 (RT-23), both of which can carry up to 10 RVs (some of them decoys, perhaps), overwhelming a point defense system becomes a game that the defender, like a slot machine addict, can’t help but lose.
And of course, no missile defense system is going to stop some lunatic from sailing a yacht into Boston Harbor or Port of Los Angeles and, if not destroying the country, creating considerable havoc and destruction. I’ve said more in this [post=5765955]thread[/post] on the topic. Suffice to say, no ABM system can legitimately make the claim of being able to defend the population, and any system developed against a strategic opponent is going to lead to a race to see who can afford more boosters, one that is far more costly for the defender than the assailant.
As for the current GMD program–it is, IMHO, a joke, a political monstrosity which exists soley for the sake of letting…um…certain individuals lay claim to be “doing something to make the nation safer” while spending less than a minimum of funds to develop and test a technically feasable system. It’s a shame, really; lacking sufficient backing and inability to set reasonable timelines means that billions of dollars spent upon the system are a waste, while deploying any system still encourages opponents to do all they can to defeat or overwhelm it. So, it doesn’t work AND it makes the situation worse. Good call.