Missile defense won't fully negate deterrent value of rogue nuclear weapons as we can never be sure it will stop all missiles
But if Tehran does obtain nuclear weapons, surrounding it with missile defenses, no matter how effective, will never eliminate the threat that a single missile could penetrate the defense system. Thus, the United States can never neutralize the deterrent value of any possible future Iranian nuclear ballistic missiles with any incarnation of missile defense. A nuclear-armed Iran would have to be treated identically by Washington whether or not missile defenses were in play.
The strategic uselessness of missile defenses aimed at intercepting nuclear-tipped missiles is clear (as I have argued before). This is a conceptual problem, not merely a technical one. The reason is simple: There is always a reasonable probability that one or more nuclear missiles will penetrate even the best missile defense system. Since a single nuclear missile hit would cause unacceptable damage to the United States, a missile defense system shouldn't change U.S. strategic calculations with respect to its enemies. Washington should treat North Korea, Iran, and other adversaries the same before and after setting up missile defense systems. Recently, Schelling publicly stated that missile defense will be of dubious value in addressing the possible future threats from Iran.