Is Iran rational? That’s the key question policy-makers and experts have been asking for at least the last decade as Iran has gotten closer to bringing its nuclear-weapons program on line.Rational, of course, is not the same thing as reasonable. A regime that shoots its own people in the streets, as the Iranian government did in June 2009, is not reasonable. In the policy debate, rationality refers to a regime’s interest in preserving itself. A regime is rational, therefore, if it understands that using a nuclear weapon would elicit a response that might spell its doom. An irrational regime is one that can’t be deterred because it may use a nuclear weapon regardless of the consequences.Thus, the Islamic Republic’s threat last week to close the Strait of Hormuz—a move that would send oil prices skyrocketing—struck many as strong evidence of the regime’s irrationality. Interrupting the world’s oil supply would compel the United States, the guarantor of Persian Gulf security, to take military actions that might mean toppling Iran’s ruling establishment. On Sunday, U.S. Joint Chief of Staff Gen. Martin Dempsey said in no uncertain terms that if Iran tries to close the Strait of Hormuz, the United States “can defeat that.”Others look at Iran’s threat to close the Strait of Hormuz as having little bearing on the country’s rationality. Since the Iranians know the Americans would have no trouble breaking through a blockade, their argument goes, Iran doesn’t actually have any intention of trying to close down one of the world’s most strategically vital waterways. This regime understands, as Defense Secretary Leon Panetta said Sunday, that closing down the Strait of Hormuz is an American red line. If Iran crosses it, it jeopardizes its own existence—and so it won’t.Those that argue the regime is irrational point to the fact that the Iranian regime regularly threatens to destroy Israel, which would retaliate by obliterating Iran. Those that claim Iran is rational write off such threats as mere rhetoric. A nuclear Iran, they say, poses little threat to a much more powerful Israel, never mind the United States. Membership in the club of countries with nuclear weapons might even make Tehran more responsible.The reality is that it doesn’t matter whether the regime is rational or not. The issue is not whether the Iranians would use the bomb, but how Tehran’s acquisition of a nuclear weapon would enhance the regime’s already reckless behavior. Moreover, it would severely limit the ability of the United States to respond to the provocations of this dangerous regime. For instance, if a nuclear-armed Iran actually closed the Strait of Hormuz, U.S. officials would be much less confident in their ability to re-open shipping lanes. American policy-makers already worried about high oil prices are not likely to risk the chances of a nuclear incident and even higher oil prices.It’s pretty easy to make a strong case that the Iranian regime really is suicidal. This is the same ruling clique, after all, that pioneered the use of the suicide car-bombing during the course of the Lebanese civil wars from 1975 to 1990. The Iranians tapped their local allies, namely Hezbollah, for martyrdom operations against Israel, the United States, and other Western powers. The Iranians spent their own blood even more recklessly in the war with Iraq when they dispatched wave after human wave of teenage boys to march through minefields, clearing a path with their bodies. Perhaps most tellingly, the plummeting Iranian birthrate—from 6.5 children per woman a generation ago to 1.7 today—suggests that it is not just the regime, but an entire nation, that no longer wishes to live.No country sets out purposefully to bring about its destruction. And yet history is nothing but the record of nations that have misunderstood the limits of their own power and the resources of their adversaries. Nazi Germany may have been suicidal, but the British Empire was not, and yet at the end of World War II both were finished. No one thinks that the rulers of Athens were irrational, but by the conclusion of the Peloponnesian War, their actions had effectively cashiered Athenian democracy.Jewish leaders between 66 C.E. and 135 C.E. were not irrational, but their revolts against Rome put an end to Jewish sovereignty for two millennia. Furthermore, who is to say that renewing Jewish sovereignty in a sea of Muslim hostility is an entirely rational act? But the rationality of any given government is irrelevant. The question of rationality moves the debate from the real to the speculative—i.e., might a given nation use the bomb at some point? The fact is no one knows beforehand whether any regime is likely to use a nuclear weapon.The only question American policy-makers should concern themselves with is whether or not a given regime seeking nuclear weapons is already hostile to U.S. interests. If it is, U.S. policy-makers should do everything in their power to prevent that regime from acquiring a bomb. The apparent injustice that Israel has the bomb while the world rues the prospect of a nuclear Iran is a quandary for academics and ethicists—and an entirely inappropriate concern for U.S. officials, whose concerns are much more specific: protecting U.S. citizens, allies, and interests. There is little debate in Washington over Israel’s nuclear-weapons program because Jerusalem has never posed a threat to American strategic interests. Iran, however, has threatened U.S. interests for 30 years.If or when Iran gets a nuclear weapon, it might drop the bomb on Tel Aviv—or Riyadh, for that matter. But that’s not the main problem. The issue is that Tehran will act in precisely the same fashion as it has since 1979—hostile to the United States and its allies—only now on a much more ambitious scale. And the range of responses available to the United States and its allies will be seriously limited.Imagine Iran with a nuclear weapon: Tehran will continue to support terror, except that Iranian assets like Hezbollah and Hamas would now be operating under a nuclear umbrella, which will shape Israeli responses. In planning its military strategy, Israel already has to take into consideration world opinion and the strain warfare puts on Israeli society and the economy. Now Jerusalem will have to wonder if crossing the border into Lebanon or Gaza will elicit nuclear threats from Iran.The Iranians will further extend their reach into Africa and Latin America, where Iranian President Mahmoud Ahmadinejad is in the midst of a regional tour. Allies like Venezuela’s Hugo Chávez will be emboldened to take otherwise unimaginable risks in Washington’s direct sphere of influence in the Americas. The recently unveiled Iranian plot to assassinate the Saudi ambassador in Washington would be only a taste of things to come.In other words: If Tehran gets a nuclear weapon, will U.S. policy-makers be prepared to ensure that the Islamic Republic doesn’t make good on a threat to close the Strait of Hormuz?Lee Smith is the author of The Consequences of Syria.