Not too many years ago, hardly anyone disagreed with John McCain when he first said that “the only thing worse than bombing Iran is letting Iran get the bomb.” Today hardly anyone disagrees with those who say that the only thing worse than letting Iran get the bomb is bombing Iran. And in this reversal hangs a tale.

The old consensus was shaped by three considerations, all of which seemed indisputable at the time.

The first was that Iran was lying when it denied that its nuclear facilities were working to build a bomb. After all, with its vast reserves of oil and gas, the country had no need for nuclear energy. Even according to the liberal Federation of American Scientists a decade ago, the work being done at the Iranian nuclear facilities was easily “applicable to a nuclear weapons development program.” Surprisingly, a similar judgment was made by Mohamed ElBaradei, the very dovish director of the International Atomic Energy Agency.

The second consideration was that the prospect of being annihilated in a retaliatory nuclear strike, which had successfully deterred the Soviets and the Chinese from unleashing their own nuclear weapons during the Cold War, would be ineffective against an Iran ruled by fanatical Shiite mullahs. As Bernard Lewis, the leading contemporary authority on Islam, put it in 2007, to these fanatics “mutual assured destruction is not a deterrent, it is an inducement. We know already [from the Iran-Iraq war] that they do not give a damn about killing their own people in great numbers. . . . They are giving them a quick free pass to heaven and all its delights.”

Iranian President Hasan Rouhani at a Nov. 24 news conference in Tehran. EPA

Nor were the rulers of Iran deterred by the fear that their country would be destroyed in a nuclear war. In the words of the Ayatollah Khomeini, who brought the Islamist revolution to Iran in 1979: “We do not worship Iran, we worship Allah. . . . I say let this land [Iran] go up in smoke, provided Islam emerges triumphant in the rest of the world.” (The quote appeared in a 1981 Iranian collection of the ayatollah’s speeches. In later editions, that line and others were deleted as Iran tried to stir up nationalistic fervor amid the war with Iraq.)

And here, speaking in particular of a nuclear exchange with Israel—that “cancer” which the mullahs were and are solemnly pledged to wipe off the map—is the famous “moderate” Hashemi Rafsanjani, in an Al-Quds Day sermon at Tehran University on Dec. 14, 2001: “Application of an atomic bomb would not leave anything in Israel, but the same thing would just produce damages in the Muslim world.” Mr. Rafsanjani, an earlier president of Iran, is the sponsor and mentor of its current president, that other celebrated “moderate,” Hasan Rouhani.

The third consideration behind the old consensus was the conviction that even if the mullahs could be deterred, their acquisition of a nuclear capability would inevitably trigger a nuclear-arms race in the Middle East. Because the Sunni regimes in Saudi Arabia, Egypt and elsewhere throughout the region were all terrified at the prospect of being lorded over and held hostage by an Iran ruled by their ancestral enemies the Shiites, those regimes would rush to equip themselves with their own nuclear arsenals.

Such an arms race would vastly increase the danger that these weapons might go off, if not by design then by accident. Retired Col. Ken Allard, a former dean of the National War College, explained why last week in the Washington Times: “Even with the steady injection of technology, U.S. and Soviet permissive-action links and fail-safe systems still needed a fair amount of luck to avoid an accidental detonation. What about Iranian, Saudi or even Egyptian nuclear forces? If they build such weapons, will they also invest in the technologies and practice the unforgiving disciplines needed to avoid the worst of all man-made calamities?”

Just as almost everyone agreed that Iran must be prevented from acquiring a nuclear capability, there was a similarly broad agreement that this could be done through a judicious combination of diplomacy and sanctions. To be sure, there were those—myself emphatically included—who argued that nothing short of military action could do the trick. But we were far outweighed by the proponents of peaceful means who, however, willingly acknowledged that the threat of military action was necessary to the success of their strategy.

Yet as the years wore on, it became clear, even to the believers in this strategy, that the Iranians would not be stopped either by increasingly harsh sanctions—or by endless negotiations. One might have expected the strategy’s proponents to conclude, if with all due reluctance, that the only recourse left was to make good on the threat of military action. Yet while they continued to insist that “all options are on the table,” it also became increasingly clear that for Western political leaders as well as the mainstream think tanks and the punditocracy, the stomach for the military option was no longer there, if indeed it had ever been.

And so began the process of what Col. Allard calls “learning to love the Iranian bomb.” The first step was to raise serious doubts about the old consensus. Yes, the Iranians were determined to build a bomb, and, yes, the mullahs were Islamist fanatics, but on further reflection there was good reason to think that they were not really as suicidal as the likes of Bernard Lewis persuaded us. That being the case, there was also good reason to drop the idea that it would be impossible to deter and contain them, as we had done even with the far more powerful Soviets and Chinese.

It was the new consensus shaped by such thinking that prepared the way for the accord reached by six major powers with Iran in Geneva last month. The Obama administration tells us that the interim agreement puts Iran on a track that will lead to the abandonment of its quest for a nuclear arsenal. But the Iranians are jubilant because they know that the only abandonment going on is of our own effort to keep them from getting the bomb.

Adherents of the new consensus would have us believe that only two choices remain: a war to prevent Iran from acquiring nuclear weapons or containment of a nuclear Iran—with containment the only responsible option. Yet as an unregenerate upholder of the old consensus, I remain convinced that containment is impossible, from which it follows that the two choices before us are not war vs. containment but a conventional war now or a nuclear war later.

Given how very unlikely it is that President Obama, despite his all-options-on-the-table protestations to the contrary, would ever take military action, the only hope rests with Israel. If, then, Israel fails to strike now, Iran will get the bomb. And when it does, the Israelis will be forced to decide whether to wait for a nuclear attack and then to retaliate out of the rubble, or to pre-empt with a nuclear strike of their own. But the Iranians will be faced with the same dilemma. Under these unprecedentedly hair-trigger circumstances, it will take no time before one of them tries to beat the other to the punch.

And so my counsel to proponents of the new consensus is to consider the unspeakable horrors that would then be visited not just on Israel and Iran but on the entire region and beyond. The destruction would be far worse than any imaginable consequences of an Israeli conventional strike today when there is still a chance to put at least a temporary halt, and conceivably even a permanent one, to the relentless Iranian quest for the bomb.

Mr. Podhoretz was the editor of Commentary from 1960-95. His most recent book is “Why Are Jews Liberals?” (Doubleday, 2009).