In his Washington Post op-ed (
Stephen Hadley: To stop Iran, stop Assad - The Washington Post), Stephen J. Hadley, former national security adviser to President Bush wrote:
Bashar al-Assad has repeatedly and flagrantly crossed a U.S. “red line” by using chemical weapons against his own people. If the United States does not take military action, how credible will be the U.S. threat to use military force if the Iranian regime continues to pursue nuclear weapons? If that threat is not credible, then only months from now our nation could face the prospect of accepting a *nuclear-armed Iran or having to resort to military force to prevent it.
That's only partially right. When it comes to deterrence, deterrence depends on a nation's possessing sufficient capacity to use force (by itself or with allies), willing to use force in the situation it is trying to deter, and its enemy's knowing that it is both willing and able to use force. If any of those elements is not present, then deterrence fails.
The first component is beyond dispute. The U.S. is the world's foremost military power among a handful of great powers. Iran lacks the individual capacity and allies to counter the U.S. The second component is also fairly certain. The U.S. has long maintained a strategic doctrine that it would act militarily, if necessary, if another nation or group of countries attempted to impede the passage of oil through the Persian Gulf or attacked Israel. That leaves the third element, Iran's perceptions, for debate.
Hadley's argument is that if the U.S. fails to respond militarily against Syria despite the President's personally having made the use of chemical weapons a "red line" would shatter U.S. credibility. Actions speak louder than words, so it might seem that Hadley is correct.
However, there's more to the story. Credibility is defined not only by a consistency between words and actions. When it comes to grave matters such as military force, more is involved. The preeminent reason nations use force, aside from self-defense, is to safeguard critical interests that, if undermined, would pose a significant threat to their own security. The same holds true with respect to strategic allies. Indeed, the disconnect between the President's red line and actual U.S. interests in Syria (small) undermined deterrence in that sectarian conflict.
That would not be the case with respect to Iran. There is no doubt that free passage through the Persian Gulf, security of friendly Gulf States, and security of strategic allies (Israel, Jordan, and Egypt) are critical U.S. interests. A nuclear-armed Iran would threaten to overturn the region's balance of power in a fashion that would pose a significant threat to all of those vital interests (directly or indirectly e.g., through Iranian proxies such as Hezbollah).
That would leave the U.S. with two major courses of action if diplomacy and sanctions fail:
1. The development of a deterrence regime in which, for example, any nuclear device used or attempted to be used against a strategic U.S. ally or in a bid to curtail use of the Persian Gulf would be met with an assured nuclear response by the U.S. that would all but eliminate Iran (M.A.D. remade). Iran is a rational actor despite its revolutionary rhetoric. Knowing that it would be held responsible even if a terrorist group used a nuclear device against let's say Israel would preclude Iran from proliferating such technology or devices. In other words, deterrence could work. Whether the U.S. would have the stomach to construct a Cold War-style deterrence regime is questionable.
2. Military strikes should Iranian possession or development of nuclear weapons become imminent.
In short, there is little reason for Iran to believe that nuclear weapons would give it the capacity to exercise regional preeminence. It would know that the U.S. would not permit it free rein, even if it possessed such weapons. It would also know that there is more than a reasonable prospect that the U.S. could act militarily to prevent it from achieving a nuclear breakout. In short, Hadley's view is but one scenario. His attempt to link Iran's calculations to whether the U.S. carries out military strikes against Syria is probably weak given the qualitative difference in U.S. interests at stake.