Nuclear Iran could use nuclear weapons to prevent regime change
If it feels its regime survival is at stake, all rational reasons deterring Iran from using nuclear weapons will no longer apply.
Quicktabs: Arguments
Additionally, one of the few scenarios where Iran might use its nuclear capability would be if Tehran believed that the United States intended to exercise forcible regime change. A nuclear strike against any American presence in the region might be seen by the leadership in Tehran as its last hope for survival. It goes without saying that once any government has crossed the nuclear threshold, forcible regime change by an external actor is no longer a viable option. The threat of nuclear retaliation would simply be too great. Indeed, this is probably the most important reason why states such as Iran and North Korea desire nuclear weapons. Does this mean that the United States should therefore seek regime change before Iran develops its nuclear capability? No; even without nuclear weapons, forcible regime change in Iran and the ensuing occupation would entail too great a commitment of resources on the part of the United States. Pursuing regime change in Iran as a response to its nuclear program would be akin to treating a brain tumor with a guillotine. The proposed cure is worse than the disease.
Such a scenario will perhaps appear manageable, if not attractive, to Western strategists. We would lament the potential loss of the option to "finish the job" with impunity against an adversary, but the "nuclear weapons as last-ditch deterrent" scenario grants the initiative to the side whose conventional forces are dominant: As long as U.S. leaders understand where the enemy's red lines lie, they can prosecute military operations up to those points and then assess the balance of risks and gains before considering their strategy for war termination. However, it would be imprudent to assume that future nuclear-armed adversaries will necessarily behave in this way. There are several reasons for this.First, adversary leaders may fear that their lives and their regimes are at grave risk from the very outset of the conflict. In Afghanistan and in Iraq, the United States demonstrated that it had the intention, if not the capability, to kill enemy leaders by bombing the buildings they were thought to be occupying. And while those particular attacks failed to kill their intended targets, the regimes themselves were overturned within a matter of weeks following the commencement of serious fighting. To the extent that U.S. forces are credited with the capability to carry out decapitating strikes or to rapidly take down enemy regimes, the belief will likely grow that one must act early to stop the U.S. military operation before it is too late.
Second is the classic use-or-lose dilemma: Adversary leaders may fear that, even if they survive U.S. bombing attacks, the United States and its allies might locate and destroy their small arsenal of nuclear weapons and delivery means before they can be brought to bear. Or if the weapons themselves are secure, the communications or other infrastructure needed to employ them effectively may be vulnerable. Concerns such as these will add to the pressures that enemy leaders will feel to escalate early. (We argue later in this chapter that U.S. counterforce capabilities against plausible regional adversaries are not impressive, but that may change over time, and, in any case, it is the perceptions of the adversary leaders that count here, not the reality.)
Third, operational considerations might also argue in favor of early use of nuclear weapons by the regional adversary. U.S. forces deploying to a distant theater in a crisis or conflict are likely to be weakest at the outset of that deployment, before the bulk of the force and its sustainment assets arrive. U.S. air and missile defenses in theater at the commencement of an operation may be thin, making it more probable that an adversary's aircraft or missiles carrying nuclear payloads will reach their targets. Also, regional adversaries may believe that some sort of nuclear "demonstration shot" could deter regional governments from granting U.S. expeditionary forces access to facilities or deter U.S. decisionmakers from prosecuting further military operations.