Much has changed in the two years since I wrote “Time to Attack Iran,” but one basic fact hasn’t: diplomacy remains unlikely to neutralize the threat from Iran’s nuclear program. A truly comprehensive diplomatic settlement between Iran and the West is still the best possible outcome, but there is little reason to believe that one can be achieved. And that means the United States may still have to choose between bombing Iran and allowing it to acquire a nuclear bomb. That would be an awful dilemma. But a limited bombing campaign on Iran’s nuclear facilities would certainly be preferable to any attempt to contain a nuclear-armed Iran.

The successful negotiation of an interim deal between Iran and the United States and its negotiating partners has not substantially improved the chances that this problem will be resolved diplomatically. On the most important issue, the two sides are as far apart as ever, at least judging from the way that the Iranian government still makes claims of a “right to enrich” uranium, despite the multiple U.N. Security Council Resolutions that have demanded the suspension of Iran’s uranium enrichment program. Any deal that permits Iran to continue enriching uranium cannot be considered comprehensive in any sense. At present, it is estimated that Iran could dash to a nuclear weapons capability in two or three months. A deal that allows limited enrichment would push that timeline back to about six months, at best. (Some analysts, including Joseph Cirincione and Colin Kahl, have misleadingly claimed that the world still has years to solve the problem because it would take Iran a long time to develop an arsenal of deliverable warheads. But that is beside the point: Whenever the Iranian government develops bomb-grade fissile material, it can then move that material to an undisclosed location, thus taking the West’s military option off the table.) In other words, the comprehensive deal under discussion would put the two sides back where they were in January 2012, when “Time to Attack Iran” was first published.

It is tempting to believe that the new atmosphere of détente between the Iranian and U.S. governments makes launching a military operation against Iran politically infeasible. In fact, a number of scenarios could trigger an attack. First, the diplomatic track might break down altogether. Congress might pass sanctions that scuttle the deal; Iranian hard-liners might do their part to undermine it; Iran’s supreme leader, Ayatollah Ali Khamenei, might be unwilling to make necessary concessions; or the diplomats might simply fail to come to mutually acceptable terms. If any of these things happen and Iran resumes its nuclear activities, Washington would then have months to either use force or prepare for a nuclear-armed Iran.

Second, diplomats might fail to produce a comprehensive deal and instead settle for making the interim deal permanent. The text of the interim deal states that it is “renewable by mutual consent,” but renewing the current deal would leave Iran’s program perpetually two or three months away from a breakout capability -- a very thin margin of error for U.S. policymakers. Any suggestion that Iran was violating the terms of the deal would have to lead to immediate consideration of a military option.

Third, if Tehran does agree to a deal that permits enrichment, it might violate the deal’s terms by quietly continuing to pursue a nuclear weapons breakout capability. Iran’s leaders would like to have sanctions relief and nuclear weapons too. At present, the U.S. government and the international community are laser-focused on Iran, but once the United States formally declares an end to the Iranian nuclear crisis, its gaze will wander. Relations will be normalized, trade will resume, and global leaders will forget about Iran and start worrying about other issues. Iran may calculate that it would be difficult for the United States to rally support for new international sanctions if Iran cheats on its agreements. In the absence of renewed international pressure, the United States would be forced to consider a military option to stop Iran from building the bomb.

Fourth, even if Iran fully abides by the terms of a deal, it would only be for a limited time. The text of the interim agreement promises that the comprehensive agreement would hold for a “specified long-term duration.” Early reports suggest that Iranian officials envision a three-to-five-year timeframe for a comprehensive accord, whereas the P5 plus 1 will press for 10 to 20 years. At the end of that specified time period, however long that might be, all bets would be off and Iran could resume its march to a nuclear weapons capability without violating the agreement.

Any discussion of a U.S. attack on Iran is sure to elicit opposition in the United States. But the White House would be wrong to heed the arguments of those who would voice moral objections to such an attack. If the rules that govern the international system, including the nuclear nonproliferation regime, are to have any meaning, they must be enforced. Some people are comfortable with military intervention for humanitarian reasons but place nuclear proliferation in a different category. Yet the spread of nuclear weapons poses a grave threat to international peace and security. If the United States believes that it is imperative to prevent nuclear war and stop additional countries from acquiring the world’s deadliest weapons, then it must be willing, in principle, to use force to achieve that objective.

When it comes to using force to prevent nuclear proliferation, the questions are practical ones: Does the use of force have a reasonable chance of success, and is it superior to available alternatives? In some instances, such as North Korea’s nuclear program today, those questions must be answered in the negative. But Iran is different. A U.S. strike, provided it is launched in time, could destroy Iran’s key nuclear facilities, set Iran’s nuclear program back a number of years, at a minimum, and, by changing a number of factors, including the calculations of Iran’s government, create a significant possibility that Iran never acquires nuclear weapons. To be sure, there are serious risks, but they pale in comparison to the dangers of living with a nuclear-armed Iran for decades to come, the further spread of nuclear weapons in the region and around the world, and an increased risk of nuclear war against Israel and the United States.

The United States must, of course, always update its assessments in light of new evidence, but nothing that has transpired in the past two years changes the fact that a military intervention may be necessary to solve the Iranian nuclear crisis. Iran began enrichment at Fordow, a facility buried in the side of a mountain near the holy city of Qom, but the facility is no match for the United States’ new and improved bunker-busting bombs. The Arab Spring toppled other governments in the region, but the Iranian regime remains strong, passing the presidency without violence or protest to a regime insider in August of this year. Iran’s new president, Hassan Rouhani, is certainly less of a firebrand than Ahmadinejad, but his election would not in any way make a nuclear-armed Iran less dangerous.

The most important change in the past two years, however, is that President Barack Obama has come out forcefully on my side of this debate and against the arguments of my critics. As he has stated many times since March 2012, a nuclear-armed Iran “not a challenge that can be contained” and the United States must be prepared to do “everything required to prevent it.” Many outside the Beltway express skepticism when Obama makes such threats, but his closest advisers insist that he is fully committed to preventing a nuclear arms race in the Middle East and is prepared to use force if necessary to keep Tehran from getting the bomb. Fortunately, the situation is not yet at that point. For now, everyone should hope for a satisfactory diplomatic resolution to the crisis. But, if that effort fails, no one, especially not Iran’s leaders, should delude themselves about what should come next.