The prospect of U.S. military action against Iran is making headlines — again.

On June 10, Sen. Joseph Lieberman, I–Conn., said that the United States must “be prepared to take aggressive military action against the Iranians.” A week earlier, during a nationally televised debate, several of the leading GOP presidential contenders refused to rule out first use of nuclear weapons against Iran.

Although Lieberman was primarily concerned with suspected Iranian activities in Iraq, most of the focus has been on Iran’s nascent nuclear program. But to advocate preventive war at this stage presumes that Iran, having successfully mastered the nuclear fuel enrichment cycle, will not only proceed to the development of nuclear weapons but will then use these weapons.

We have heard such fatalism before.

Former CIA Director James Woolsey predicted in 1993 that Iran would have a nuclear bomb within a decade. Michael Ledeen of the American Enterprise Institute warned in April 2003 that Iran might conduct a nuclear test that summer. Columnist Charles Krauthammer warned in January 2006 that Iran was “probably just months away” from the point of no return in its nuclear program.

Admittedly, achieving a verifiable end to Iran’s nuclear ambitions is a difficult challenge. There are no magical solutions. Policymakers must choose from a set of imperfect options, but they must avoid a sense of panic. There is no imminent threat. We have time to carefully weigh the likely costs and prospective benefits of each of the four main options on the table.

Option One: Sanctions

Sanctions have been a key component of U.S. policy since the Islamic Revolution of 1979. Historically, sanctions have a poor track record. They tend to hurt the very people most inclined to push a regime toward political reform, and with the possible exception of the South African apartheid regime in the 1980s, they almost never persuade a targeted government to abandon high–priority policies.

Sanctions spring from an impulse to do something regardless of its usefulness. The U.N. Security Council has approved two rounds of multilateral sanctions on Iran within the past nine months, so far with little success.

If the Security Council managed to pass a broad–ranging sanctions package that included the one product on which Iran is so heavily dependent (oil), and if it succeeded in rigidly enforcing the system, the likely result would be skyrocketing world oil prices, making smuggling prohibited goods even more profitable. Driving the market further underground is likely to enrich Iran’s rulers at their citizens’ expense.

Option Two: Subversion

Under the Iran Freedom Support Act, the United States has been funding Iranian opposition groups with the goal of undermining the government in Tehran. The program’s actual scope is almost laughably modest — $75 million in the 2007 budget — but the precedent set by the similarly modest Iraq Liberation Act should give anyone pause.

Undoubtedly, many young Iranians are fed up with repressive Islamist rule and want a more open society, but U.S. support for democratic–minded reformers allows the theocracy to claim that the U.S. is meddling in Iranian domestic affairs.

As Ted Galen Carpenter has noted, “A good many Iranians remember that the U.S. interfered once before in their country’s internal affairs, and the result was not a happy one.” In this context, reformers are branded as traitors and American stooges. No wonder, then, that prominent Iranian dissidents Shirin Ebadi and Muhammad Sahimi recently wrote that “no truly nationalist and democratic group will accept such funds.”

Option Three: Military Action

But if sanctions are likely to fail, and if subversion might prove counterproductive, military action is the least desirable of all.

The costs of military action are almost certain to vastly exceed the benefits. What’s more, even the advocates of military strikes believe that such attacks will merely postpone — not eliminate — the Iranian nuclear program.

Many pro–war analysts argue that the strikes can be limited to Iran’s nuclear facilities and therefore will not devolve into an Iraq–style quagmire, but there are significant problems associated with this approach.

First, Iran’s nuclear facilities are well hidden. A successful attack would require first–rate intelligence, and the House Select Committee on Intelligence concluded in 2006 that “American intelligence agencies do not know nearly enough about Iran’s nuclear program.”

Many Iranian nuclear facilities have been hardened against attack, with some buried under as much as 18 meters — 59 feet — of rock and concrete. Since our most effective conventional “bunker–busters” can only penetrate 6 meters — 20 feet — underground, the U.S. military might have to resort to low–yield nuclear weapons to bust the bunkers. Also, because many of the facilities are located in or near major population centers, bombing would result in significant civilian casualties.

Whether an attack used nuclear or conventional munitions, it probably would lead to a wider war.

Iranian leaders probably would conclude that the survival of the regime was at stake, and they would have no incentive to hold anything back. They could retaliate in several ways.

Although Iranian–sponsored terrorist attacks against the United States itself are unlikely, they cannot be ruled out entirely. More likely are attacks against U.S. interests in the region. Attacking fragile U.S. supply lines that snake from Kuwait to Baghdad through heavily Shiite southern Iraq would be a low–risk, high–reward strategy for Tehran.

Then there is the possibility that Iran would shut down the Strait of Hormuz. Even the threats from mines and small suicide boats are sufficient to give military planners pause.

And although the Iranians might be doing themselves long–term harm by cutting off or severely restricting the flow of oil, their chief source of state income, even minor disruptions in the global oil market often result in short–term windfalls for oil exporters. Cambridge Energy Research Associates estimated in July 2006 that even a $5 increase in the price of a barrel of crude would generate an additional $85 million per week in revenues for Iran.

A likely unintended consequence of a military strike against Iran would be a potentially fatal blow to Iran’s nascent political reform movement. Some of the leading proponents of military strikes against Iran hope that the Iranian people, under the stress of devastating aerial bombardment, will embrace the very people dropping bombs on them.

A “rally–’round–the–flag effect” that strengthens hard–liners and marginalizes reformers is far more likely.

It is nearly impossible to find Iranian dissidents who favor even targeted U.S. military strikes against their country. Shirin Ebadi has flatly warned that “any attack on Iran will be good for the government.” The Hoover Institution’s Abbas Milani, a founder of the Iran Democracy Project, agrees: “An American or Israeli attack on Iran would sound the death knell of [the democratic] movement.”

Option Four: Deterrence

Deterrence has been tried before. The United States managed to deter some unpleasant people, including Josef Stalin and Mao Zedong, from using nuclear weapons during the Cold War.

Could we not also deter the mullahs from engaging in suicidal behavior? Simply put, are the mullahs crazy?

Not at first glance. As Cato Institute foreign policy analyst Justin Logan documents in a recent paper, Iran’s leaders have exhibited moments of prudence and pragmatism during the past 27 years, and they have even reversed course when confronted with overwhelming force.

As the Iran–Iraq War dragged into its eighth year, the costs of fighting on became too high, Ayatollah Ruhollah Khomeini acted rationally. Though his earlier rhetoric had been uncompromising, the leader of the Iranian revolution sued for peace when faced with the prospect of complete and total destruction by the hated Iraqis, who were then backed by the equally hated United States.

Khomeini’s successors would suffer a vastly greater defeat if they were foolish enough to launch nuclear weapons against the United States. They are almost certainly not that foolish.

The slightly more plausible prospect — that Iran would give nuclear weapons to a terrorist proxy, believing that it could shift the risks to others and avoid paying the ultimate price — is also highly unlikely because such a transfer would not advance Iran’s strategic interests. What’s more, the mullahs could not ensure that the weapons would only be used against their enemies. Iran has never been known to transfer chemical or biological weapons to its longtime proxy Hezbollah, and the transfer of nuclear weapons would be far more risky.

Deterrence carries its own risks. We had a few close calls during the Cold War. And we have yet to test our ability to deter states from transferring weapons to non–state actors. It is useful, however, to weigh the costs and risks of deterrence against the much higher costs and risks of preventive war.

Option Five: Bargain

There is one other option available to policymakers, however: Washington could offer a grand bargain to Tehran.

The Bush administration would renounce its stated policy of regime change in Iran and would offer to normalize diplomatic and trade relations. In exchange, Tehran would pledge to open its nuclear program to rigorous, on–demand international inspections to guarantee that nuclear material was not used for weapons purposes.

If the Iranians rejected such an offer, we could revert to alternative policies. If the theocracy endured, or if a new regime came to power but continued to move ahead with the nuclear program, the United States should be prepared to rely on deterrence.

The one thing we must not do is start another preventive war. That could engulf the entire region and threaten U.S. interests for many years to come.