IN 2009 Iran was on the verge of electing a reformer as president. Ayatollah Ali Khamenei, the supreme leader, subverted the vote and crushed the ensuing protests. Last week the same desire for change handed a landslide victory to Hassan Rohani—and Mr Khamenei hailed it as a triumph.

When a country has seen as much repression as Iran, outsiders hoping for a better future for the place instinctively want to celebrate along with all those ordinary Iranians who took to the streets. The smiling Mr Rohani’s public pronouncements encourage optimism, for he sounds like a different sort of president from the comedy-villain, Mahmoud Ahmadinejad, who precedes him. Yet even if his election bodes well for Iranians, it does not necessarily hold equal promise for the rest of the world. Iran’s regional assertiveness and its nuclear capacity mean that it is a more dangerous place than it ever was before.

Given the country’s obvious weaknesses, that sounds implausible. Inflation is running at over 30%, and the economy shrinking. Inequality is growing, with 40% of Iranians thought to be living below the poverty line. Sanctions restricted May’s oil exports to just 700,000 barrels a day, a third of what they used to be; as a result there are shortages of basic goods and growing unemployment caused by factory closures.

Yet the Persian lion has not lost its claws, nor has the theocracy suddenly become a democracy. Mr Rohani was indeed the most reformist of the candidates on offer at the election, but in much the way that Churchill was more of a teetotaller than George Brown. The 64-year-old cleric has been a loyal servant of the Islamic Republic from its inception. For years he headed the national security council (see article). He is constrained by a system that deemed just eight people fit to stand in the recent election and rejected 678 others (including a former president). The president’s power is limited by Iran’s other institutions, many of which are in conservative hands.

While Iran’s politics have probably changed less than Mr Rohani’s election suggests, the balance of power between Iran and the rest of the world has been shifting in Iran’s favour for two reasons. First, thanks to heavy investment in nuclear capacity by the mullahs, and despite attempts by the West and Israel to delay or sabotage the nuclear programme, Iran will soon be able to produce a bomb’s worth of weapons-grade uranium in a matter of weeks (see briefing). Iran has installed more than 9,000 new centrifuges in less than two years, more than doubling its enrichment capability. It is a short step from the 20% enriched uranium that the country’s facilities are already producing at an increasing rate to conversion into the fissile material needed for an implosion device. Although Western intelligence agencies think Iran is still at least a year away from being able to construct such a weapon, some experts believe that it could do so within a few months if it chose to—and that the time it would take is shrinking.

This makes a nonsense of Western policy on Iran. Round after round of negotiations to try to persuade Iran not to get a bomb have been backed up by the implicit threat that armed force would be used if talks failed. But now it looks as though Iran will soon be in a position to build a weapon swiftly and surreptitiously. Should the West decide to use force, Iran could amass a small arsenal by the time support for a military strike was rallied.

Against that background, a friendlier president becomes a trap as well as an opportunity. He may offer the chance of building better relations through engagement and the gradual lifting of sanctions. But Iran could take advantage of this inevitably slow process to build a weapon.

The other development that threatens the West’s interests is happening around Iran. Despite its economic troubles, the Iranian state is a powerful beast compared with its neighbours, and is keen to assert itself abroad. The Iraqi government is now its ally. It has sway over chunks of Lebanon through Hizbullah, the Shia party-cum-militia it finances. And it has sent Hizbullah into Syria, where its fighters have joined Iranian advisers, money and special forces to help turn the tide of the war in Bashar Assad’s favour. Ostensibly the reason why Barack Obama agreed last week to arm the rebels in Syria (see article) was Mr Assad’s use of chemical weapons; but many believe that the greater reason was his reluctance to see Mr Assad hold on to power as a client of Iran’s.

Hope for the best, prepare for the worst

This analysis may be too gloomy. It is possible that Mr Rohani’s arrival heralds a more pragmatic and less aggressive position. The new president used to serve as Iran’s main nuclear negotiator, and during his campaign made clear the link between Iran’s economic weakness and the nuclear sanctions, and called for better relations with the West. The West should reciprocate, making it clear that it has no intention of impeding Iran’s peaceful development. At the same time, it should continue to push for progress on the nuclear negotiations.

But it must do so warily. Any deal offered to Iran should include restraints draconian enough, and inspection intrusive enough, to prevent it from building a weapon surreptitiously, otherwise it would be worse than not doing a deal at all. And such a deal would very likely be unacceptable to Iran.

The growing risk of a nuclear Iran is one reason why the West should intervene decisively in Syria not just by arming the rebels, but also by establishing a no-fly zone. That would deprive Mr Assad of his most effective weapon—bombs dropped from planes—and allow the rebels to establish military bases inside Syria. This newspaper has argued many times for doing so on humanitarian grounds; but Iran’s growing clout is another reason to intervene, for it is not in the West’s interest that a state that sponsors terrorism and rejects Israel’s right to exist should become the regional hegemon.

The West still has the economic and military clout to influence events in the region, and an interest in doing so. When Persian power is on the rise, it is not the time to back away from the Middle East.