FOR years Iran has practised denial and deception; it has blustered and played for time. All the while, it has kept an eye on the day when it might be able to build a nuclear weapon. The world has negotiated with Iran; it has balanced the pain of economic sanctions with the promise of reward if Iran unambiguously forsakes the bomb. All the while, outside powers have been able to count on the last resort of a military assault.

Today this stand-off looks as if it is about to fail. Iran has continued enriching uranium. It is acquiring the technology it needs for a weapon. Deep underground, at Fordow, near the holy city of Qom, it is fitting out a uranium-enrichment plant that many say is invulnerable to aerial attack. Iran does not yet seem to have chosen actually to procure a nuclear arsenal, but that moment could come soon. Some analysts, especially in Israel, judge that the scope for using force is running out. When it does, nothing will stand between Iran and a bomb.

The air is thick with the prophecy of war. Leon Panetta, America's defence secretary, has spoken of Israel attacking as early as April. Others foresee an Israeli strike designed to drag in Barack Obama in the run-up to America's presidential vote, when he will have most to lose from seeming weak.

A decision to go to war should be based not on one man's electoral prospects, but on the argument that war is warranted and likely to succeed. Iran's intentions are malign and the consequences of its having a weapon would be grave. Faced by such a regime you should never permanently forswear war. However, the case for war's success is hard to make. If Iran is intent on getting a bomb, an attack would delay but not stop it. Indeed, using Western bombs as a tool to prevent nuclear proliferation risks making Iran only more determined to build a weapon—and more dangerous when it gets one.

A shadow over the Middle East

Make no mistake, an Iran armed with the bomb would pose a deep threat. The country is insecure, ideological and meddles in its neighbours' affairs. Both Iran and its proxies—including Hizbullah in Lebanon and Hamas in Gaza—might act even more brazenly than they do now. The danger is keenly felt by Israel, surrounded by threats and especially vulnerable to a nuclear bomb because it is such a small land. Iran's Supreme Leader, Ayatollah Ali Khamenei, recently called the “Zionist regime” a “cancerous tumour that must be cut out”. Jews, of all people, cannot just dismiss that as so much rhetoric.

Even if Iran were to gain a weapon only for its own protection, others in the region might then feel they need weapons too. Saudi Arabia has said it will arm—and Pakistan is thought ready to supply a bomb in exchange for earlier Saudi backing of its own programme. Turkey and Egypt, the other regional powers, might conclude they have to join the nuclear club. Elsewhere, countries such as Brazil might see nuclear arms as vital to regional dominance, or fear that their neighbours will.

Some experts argue that nuclear-armed states tend to behave responsibly. But imagine a Middle East with five nuclear powers riven by rivalry and sectarian feuds. Each would have its fingers permanently twitching over the button, in the belief that the one that pressed first would be left standing. Iran's regime gains legitimacy by demonising foreign powers. The cold war seems stable by comparison with a nuclear Middle East—and yet America and the Soviet Union were sometimes scarily close to Armageddon.

The dream of pre-emption

No wonder some people want a pre-emptive strike. But military action is not the solution to a nuclear Iran. It could retaliate, including with rocket attacks on Israel from its client groups in Lebanon and Gaza. Terror cells around the world might strike Jewish and American targets. It might threaten Arab oil infrastructure, in an attempt to use oil prices to wreck the world economy. Although some Arab leaders back a strike, most Muslims are unlikely to feel that way, further alienating the West from the Arab spring. Such costs of an attack are easy to overstate, but even supposing they were high they might be worth paying if a strike looked like working. It does not.

Striking Iran would be much harder than Israel's successful solo missions against the weapons programmes of Iraq, in 1981, and Syria, in 2007. If an attack were easy, Israel would have gone in alone long ago, when the Iranian programme was more vulnerable. But Iran's sites are spread out and some of them, hardened against strikes, demand repeated hits. America has more military options than Israel, so it would prefer to wait. That is one reason why it is seeking to hold Israel back. The other is that, for either air force, predictions of the damage from an attack span a huge range. At worst an Israeli mission might fail altogether, at best an American one could, it is said, set back the programme a decade (see article).

But uncertainty would reign. Iran is a vast, populous and sophisticated country with a nuclear programme that began under the shah. It may have secret sites that escape unscathed. Even if all its sites are hit, Iran's nuclear know-how cannot be bombed out of existence. Nor can its network of suppliers at home and abroad. It has stocks of uranium in various stages of enrichment; an unknown amount would survive an attack, while the rest contaminated an unforeseeable area. Iran would probably withdraw from the Nuclear Non-Proliferation Treaty, under which its uranium is watched by the International Atomic Energy Agency. At that point its entire programme would go underground—literally and figuratively. If Iran decided it needed a bomb, it would then be able to pursue one with utmost haste and in greater secrecy. Saudi Arabia and the others might conclude that they, too, needed to act pre-emptively to gain their own deterrents.

Perhaps America could bomb Iran every few years. But how would it know when and where to strike? And how would it justify a failing policy to the world? Perhaps, if limited bombing is not enough, America should be aiming for an all-out aerial war, or even regime change. Yet a decade in Iraq and Afghanistan has demonstrated where that leads. An aerial war could dramatically raise the threat of retaliation. Regime change might produce a government that the West could do business with. But the nuclear programme has broad support in Iran. The idea that a bomb is the only defence against an implacable American enemy might become stronger than ever.

Get real

That does not mean the world should just let Iran get the bomb. The government will soon be starved of revenues, because of an oil embargo. Sanctions are biting, the financial system is increasingly isolated and the currency has plunged in value. Proponents of an attack argue that military humiliation would finish the regime off. But it is as likely to rally Iranians around their leaders. Meanwhile, political change is sweeping across the Middle East. The regime in Tehran is divided and it has lost the faith of its people. Eventually, popular resistance will spring up as it did in 2009. A new regime brought about by the Iranians themselves is more likely to renounce the bomb than one that has just witnessed an American assault.

Is there a danger that Iran will get a nuclear weapon before that happens? Yes, but bombing might only increase the risk. Can you stop Iran from getting a bomb if it is determined to have one? Not indefinitely, and bombing it might make it all the more desperate. Short of occupation, the world cannot eliminate Iran's capacity to gain the bomb. It can only change its will to possess one. Just now that is more likely to come about through sanctions and diplomacy than war.