Iran has multiple options to retaliate against a military strike
A military strike against Iranian nuclear facilities would have multiple negative consequences, not the least of which is that it would initiate a prolonged war with Iran.
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Iran is liable to take countermeasures against the US following an attack on its nuclear installations. Retaliatory measures could include terrorist attacks against American targets, both inside and outside of the Middle East; terrorist attacks and military measures against American allies; attacks employing conventional weapons against American forces in the Gulf region or in Iraq, including a cruise missile attack on US ships; and an intensified effort to disrupt American efforts in Iraq, where Iran has influence among Shiite organizations. Iran is also liable to respond to a US attack with an attempt to hit Israeli targets, even if Israel is not involved in the attack. At the same time, the US has greater deterrence vis-à-vis Iran than Israel does, including second strike capability following Iranian retaliation. For that reason, it can be assumed that if Iran decides to respond to an American attack, its response will be restrained and careful, in order to avoid dragging the US into an escalating response. In any case, an Iranian military response against American targets is expected to lead to an American counter-response, and to a chain of actions and counteractions.
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Last, policymakers must recognize that future airstrikes may incur much higher costs for the United States than did the 1981 attack for the Israelis. Such attacks against Iran would substantially undermine U.S./Iranian relations, perhaps leading to increased terrorism, to disruptions in the world oil market, or to an Iranian decision to intervene in Iraq in support of the Shiites. The CIA reported in 2004 that Hizballah ‘‘would likely react to an attack against it, Syria, or Iran with attacks against U.S. and Israeli targets worldwide.’’66 Striking North Korean nuclear facilities could be even worse; as General Gary Luck put it, ‘‘If we pull an Osirak [against North Korea], they will be coming south.’’67 American policymakers may be better off relying on more peaceful means of counterproliferation, such as diplomacy, inspections, and economic sanctions. These tools promise fewer costs and dangers and have demonstrated more success than military action.
"Preventive Attacks against Nuclear Programs and the "Success" at Osiraq
." Nonproliferation Review
. Vol. 12, No. 2 (July 2005): 355-371. [ More (5 quotes) ]
Iran most definitely is not. Indeed, in part because Washington obligingly removed the main regional strategic counterweight to Iran by ousting Saddam, Tehran’s power and influence has been on the rise for the past decade. It has significant allies and clients in the region, especially among its Shiite brethren. Hezbollah is the most notable one, but pro-Iranian elements in Syria, Yemen, and Bahrain are not trivial assets. Tehran could make life in the Middle East even more miserable for the United States than it is currently, and the clerical regime would have every incentive to do so in response to air strikes on its nuclear installations.
The main fallacy in the professed hawkish case for limited war is that the United States would be in complete control of the escalation process. The reality is that Iran could greatly escalate the military confrontation. And one cannot imagine people in Congress like Tom Cotton, John McCain, and Lindsey Graham (or the warhawk contingent in the news media) keeping their cool and adhering to a limited war agenda if Tehran or its clients retaliated by attacking a U.S. ally in the Middle East—much less if such an attack killed Americans.
The instant that such an incident occurred, the current proponents of a limited war would demand a full-scale U.S. military campaign to overthrow the Iranian government. And at that point, we would be in a new Middle East war that would make the Iraq and Afghanistan conflicts look like minor skirmishes.
There is no question that Iran will respond to a U.S. attack, probably in ways that America’s Middle East cheerleaders (primarily in the Gulf States) won’t expect or appreciate.
Iran has a vast array of short range ballistic missiles and land attack cruise missiles.  While analysis of these missiles has focused on their potential threat to American naval vessels, they could pose a much more serious problems for ground installations around the Gulf, including both military and economic targets. Attacks on oil installations could have a wide effect on global oil prices, although the global economy is certainly better suited to weathering price shocks now than five or ten years ago. Of course, the Iranians surely appreciate that missiles, once fired, cannot be launched again, and that any such attacks run the risk of incurring escalation from the United States. Iran can also attempt to close the Straits of Hormuz through mining, small boat, and missile attacks, although any serious effort in this regard would expose Iranian military forces to near total attrition.
Iran’s political options are more intriguing than its strictly kinetic tools. When the United States removed the Baathist regime in Iraq, it effectively conceded Iranian influence on the country. Tehran will undoubtedly step up its efforts to destabilize both Iran and Afghanistan. Iran is unlikely to begin supporting ISIS, but it can certainly curtail the cooperation we’ve seen over the past months. It remains well within Iranian capabilities to increase support for Iraqi Shiite militias, as well as for the Assad regime.
The broader political environment will also shift against the United States. U.S. relations with Russia have gone sufficiently south, and the U.S. attack against Iran itself would be sufficiently destabilizing, that we can almost surely expect Russia to militarily support Iran in the form of aircraft and air defense systems. These systems would, in the medium and long term, make Iranian defenses considerably more lethal, and its nuclear program far more robust. Moreover, if Russia opens up the Iranian defense market, we can expect China to follow. The sanctions regime cannot survive a U.S. attack on Iran.
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When imposing the sanctions fails to alter Tehran's position, policymakers will revert to a strike on Iran's nuclear facilities. One can imagine the words of a planner in the meeting: "If we are going to do this, let's make certain we get everything they have." I have done some rough "targeting" of nuclear facilities for which I can find satellite photos on the Web. By my calculation, an attack of relatively high certainty on nuclear targets would require 400 aim points. (An aim point is the specific location where an individual weapon is directed. Most targets would have multiple aim points.) I estimate seventy-five of these aim points would require penetrating weapons. But it is unlikely that a U.S. military planner would want to stop there. Iran probably has two chemical weapons production plants. He would want to hit those. He would want to hit Iran's medium-range ballistic missiles that have just recently been moved closer to Iraq. There are fourteen airfields with sheltered aircraft. Although the Iranian Air Force is not much of a threat, some of these airfields are less than fifteen minutes flying time from Bagdhad. Military planners would want to eliminate that potential threat. The Pentagon would want to hit the assets that could be used to threaten Gulf shipping. That would mean targeting cruise missile sites, Iranian diesel submarines, and Iranian naval assets.
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Finally, there is the probable impact on the rest of the Muslim world. If the United States attacks yet another Muslim country (which would make three in the last five years), most Muslims from Morocco to Malaysia will believe that Washington is out to destroy their culture and religion. America's troubles with the Islamic world do not yet constitute a war of civilizations, but attacking Iran could well produce that result. The military option is one that no rational U.S. policymaker should embrace.
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The former CIA officer, Reuel Marc Gerecht wrote one of the best discussions of the expected consequences of a military campaign to seriously degrade Iran's nuclear capabilities, in the April 24, 2006 issue of The Weekly Standard. Gerecht's forthright analysis concludes that "bombing the nuclear facilities once would mean we were declaring war on the clerical regime…we'd have to strike until they stopped…. All of this would probably transpire over many years, perhaps a decade or more." [p. 23]. Despite the long war that he anticipates would follow strikes on Iran's nuclear installations and assorted other military targets, Gerecht believes the risk of allowing Iran's ruling clerics to possess nuclear weapons is greater. Retired U.S. Air Force Colonel, Sam Gardiner, an experienced conductor of military war games, argues in "The End of The ‘Summer of Diplomacy': Assessing U.S. Military Options on Iran," that the most probable military scenario will "be unlikely to yield any of the results that American policymakers do want, and…would be highly likely to yield results that they do not."
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Moreover, much of the public debate regarding preventive action has focused on military-technical considerations: Does the United States (or Israel) have the intelligence needed to hit the right targets? Does either have the means to destroy those parts of the nuclear infrastructure located in hardened, buried facilities? Is there an optimal moment to strike, and when is it too late?These questions, however, are not the primary questions that need to be asked, and they highlight the fact that the accepted wisdom is based on an inappropriate metric for measuring the success of preventive action: the amount of destruction visited upon Iran's nuclear infrastructure may matter less than whether or not Iran decides to rebuild.The accepted wisdom also ignores context: preventive action that follows provocative Iranian steps, such as an announcement that it is leaving the Nuclear Nonproliferation Treaty (NPT), could have a much different effect than action not linked to a perceived Iranian provocation. The accepted wisdom is also based on assumptions not grounded in the Islamic Republic's track record of retaliation after military attack, which is decidedly mixed. Nearly all retaliatory options entail considerable challenges and risks for Iran.
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While the popular scenario of a suicidal regime in Tehran attacking Israel as soon as it is technologically able to do so is undoubtedly oversimplified, there are nevertheless other more plausible scenarios under which the Iranians might consider such an assault. The most likely of these scenarios involves a crisis between the United States and Iran. Since the US homeland is beyond the reach of current and projected Iranian strategic platforms, Tehran can only deter the United States with its missiles and aircraft by threatening allies or US forces deployed in the region. Israel would be a natural target for Iran in any scenario involving US-Iranian tensions, followed by a possible breakdown of deterrence and the onset of war. Additionally, if the Iranians believed some future US attack against them was actually an effort at regime change, they would be more likely to consider striking the Israelis with available systems. In such a scenario the Tehran leadership might view itself as having little to lose.