U.S. intelligence officials have said they believe Iran already has the know-how and much of the technology to build a nuclear weapon. U.S. and Israeli intelligence officials agree that Iran’s leaders have not yet made a decision to build one. But the U.S. government has indicated that if Iran were to produce weapons-grade enriched uranium and build a weapon, the military option must be considered. Defense Secretary Leon Panetta said this month that the United States would have “a little more than a year . . . to take the action necessary” should Iran decide to make a dash for a nuclear weapon. We believe that there would be sufficient warning time to decide how to respond.

Though not the only way to achieve these objectives, a U.S. attack would demonstrate the country’s credibility as an ally to other nations in the region and would derail Iran’s nuclear ambitions for several years, providing space for other, potentially longer-term solutions. An attack would also make clear the United States’ full commitment to nonproliferation as other nations contemplate moves in that direction.

The costs are more difficult to estimate than the benefits because of uncertainty about the scale and type of Iran’s reaction. Iran is likely to retaliate directly but also to pursue an asymmetrical response, including heightened terrorist activity and covert operations as well as using surrogates such as Hezbollah. An increase in the price of oil could keep the market unstable for weeks or months and disrupt the global economy.

The conflict could also escalate into a regional war involving Syria, Hezbollah, the Palestinians and other Arab states and terrorist groups. While a U.S.-led attack on Iran might be quietly welcomed by the leaders of many Arab states, and certainly by Israel, it would most likely be greeted with hostility from wide swaths of the region’s Muslims.

Other consequences might include the increased likelihood of a decision by Iran to build a nuclear weapon; more instability in a region still seeking its footing; and the opportunity for extremist groups such as al-Qaeda to attract recruits.

When he accepted the Nobel Peace Prize, President Obama wisely described the dilemma that the United States faces as a great nation: “part of our challenge is reconciling these two seemingly irreconcilable truths — that war is sometimes necessary, and war at some level is an expression of human folly.” The United States needs to have a nonpartisan, reasoned discussion about the choice between necessity and human folly.