While conservatives have raised all kinds of objections to last week's vanilla arms-control agreement, it's interesting that the emotional core of their critique has been an attack on something called the "offense-defense linkage"--a wonky way of saying that that Barack Obama might agree to limitations on missile-defense deployment in return for Russian concessions on nuclear arms. This is an obscure thing to get huffy about--why not complain that the Russians will have to destroy fewer missile launchers than we will? Or that they still haven't put their easily stolen tactical nuclear arms on the table?--but "offense-defense" has made conservatives very angry indeed. "Obama's hunger for a diplomatic success, such as it is, allowed the Russians to exact a price: linkage between offensive and defensive nuclear weapons," wrote Charles Krauthammer. "Obama doesn't even seem to understand the ramifications of this concession." Speaking to the Christian Science Monitor, John Bolton zeroed in on the same issue.

There's a reason for this: Conservatives are afraid that the START follow-on agreement will resurrect a modest version of the 1972 Anti-Ballistic Missile Treaty, which outlawed ballistic missile defenses--a pact that they tried to kill for thirty years and finally succeeded in nullifying under George W. Bush. As Frances Fitzgerald and our own Peter Scoblic have explained, the conservative movement has adopted missile defense as the ultimate symbolic guarantor of America's ability to act unilaterally. If we have missile defense, the thinking goes, America can remain an exceptional nation that does not have to rely on other countries to guarantee our security: We wouldn't have to ask for Russia's help to end Iran's nuclear program, because we'd be able to shoot down Iranian ballistic missiles any time they sail toward us. The flip side of this equation, as Lawrence Kaplan once explained in TNR's own pages, is that ballistic-missile defenses allow America to act unilaterally anywhere in the world, invading small countries without worrying about retaliatory missile strikes against our own shores: "'Ballistic missile defense is not simply a shield but an enabler of U.S. action.' ... It's not about defense. It's about offense." The possibility that Obama might reintroduce the "offense-defense linkage" is a grave threat to this vision.

The only problem, of course, is that a perfect missile defense is unrealistic. Like the Maginot Line and other holy grails of military strategy (or were they green lights on the dock?), the national missile defenses we're deploying simply cannot work well enough to prevent us from being deterred by intercontinental ballistic missile technology. This system can easily be overwhelmed by decoys or extra missiles, circumvented using alternate means of delivery, or fail simply because it's unreliable. Some of the anti-missile interceptors haven't even been tested, for God's sake. Ronald Reagan's vision of a "shield that could protect us from nuclear missiles just as a roof protects a family from the rain" remains as Hollywood as ever.

So, in the limited amount of time that we have to stop Iran's nuclear program, we'd be better off trying to protect ourselves from the Iranians the old-fashioned way--by promising the Russians things they're truly interested in as a reward for pressuring Iran; squeezing Ayatollah Ali Khamenei with sanctions; offering a more amicable path to resolution of our differences; and credibly threatening to bomb.

--Barron YoungSmith