As calls mount, especially in Israel, for military action against Iran´s nuclear program, the main counterargument has been seductively simple: Iran is rational. Indeed, our country´s top military official, Army Gen. Martin E. Dempsey, recently rejected the need for airstrikes because, as he put it, "We are of the opinion that the Iranian regime is a rational actor."

By this logic, we should not risk war to prevent Iran from going nuclear because even if Iran acquired nukes, it would never use them offensively, never share them with terrorists and never utilize them as a shield for regional adventurism. To do so would risk nuclear retaliation, which would be irrational.

Although I disagree with the general´s conclusion that we shouldn´t take action, I do believe his underlying assumptions are mainly right. The Iranian regime is mostly rational most of the time. Its rhetoric is blustery, but its actions typically are moderated to avoid provoking retaliation.

For example, Iran has so far avoided kicking out international inspectors and launching a crash program to build nuclear weapons, the steps most likely to provoke airstrikes. Instead, Iran permits inspectors to verify that it is enriching uranium to a significant degree, in direct contravention of U.N. Security Council resolutions, in amounts for which it has no civilian need but that would facilitate a bomb program.

Although this strategy has provoked economic sanctions, it has also permitted Iran to both proceed steadily toward a nuclear weapons capability and avoid military retaliation. According to the latest inspection, Iran is producing enough 20%-enriched uranium each year to fuel its one nuclear research reactor for 15 years, which would make no sense. But that production rate is also sufficient for one bomb per year, if further enriched for just a few weeks, which suggests a perfectly rational strategy.

The problem is that Iran does not always act quite so rationally. Rarely, but repeatedly over the years, it has launched attacks that seemed to invite massive retaliation, for apparently little gain. Iran´s targets have included the U.S. Embassyand Marine barracks in Lebanon in 1983, the Israeli Embassy and a Jewish community center in Argentina in the early 1990s, and the U.S. military´s Khobar Towers in Saudi Arabia in 1996. Just last year, the Iranians were behind a botched scheme to assassinate the Saudi ambassador in the United States.

Some might argue that even these attacks were rational because Iran avoided massive military retaliation. But that was partially luck, and in any case Iran did suffer significant economic and diplomatic punishment. By any objective measure, the Iranian regime ran risks that greatly exceeded expected material benefits, the very definition of irrationality.

We don´t know exactly why Iran acts so irrationally from time to time. One possibility is that the regime itself is rational but lacks full control, so that extremist factions act autonomously on occasion. Another is that domestic politics drive the regime to appease extremist factions from time to time. Or it´s possible that the regime´s own radical Islamist ideology sometimes overwhelms its rationality.

Whatever the reason, the reality is that Iran seems to act rationally most — but not all — of the time. This has two major strategic implications.

First, if Iran acquired nuclear weapons, it probably would not use them, share them or ramp up regional aggression with them. Yet there is a nontrivial chance — let´s call it 5% — that Iran would utilize its nuclear weapons in such an aggressive fashion.

Second, if Israel or the United States launched surgical strikes on Iran´s key nuclear facilities, Iran probably would act rationally by not retaliating broadly against U.S. interests, which would risk provoking a major U.S. military escalation that could end the Iranian regime. Yet there is a nontrivial chance — again, perhaps 5% — that Iran would retaliate in such a broad manner, drawing the United States into a larger military conflict.

This clarifies the strategic choice for the United States: a small chance of Iran using nuclear weapons offensively in the future if we don´t launch airstrikes fairly soon, or a small chance of escalated conventional war with Iran in the near term if we do.

Is it a 2% chance? A 10% chance? We can´t know; but we do know, based on Iran´s past irrational actions, that it´s not zero.

So, which of the two should President Obama choose? The small chance of an escalated conventional war against Iran, in which we would enjoy overwhelming military superiority? Or a similarly small, but significant, chance that Iran would use nuclear weapons aggressively, inflicting massive casualties?

Is there really any question?

Alan J. Kuperman teaches military strategy at the LBJ School of Public Affairs, University of Texas at Austin, where he also coordinates the Nuclear Proliferation Prevention Project (www.nppp.org). (Copyright © 2012 Los Angeles Times 04/02/12)