Today the Obama administration begins the most important American diplomatic engagement with Iran since the 1979 Islamic Revolution. The public revelation of the Qom enrichment facility and Iran’s provocative ballistic missile test on Monday demonstrate what is at stake. Iran has increased its low-enriched uranium 20-fold since 2007, enough to produce, eventually, at least one nuclear weapon after further enrichment.

These are ominous developments. But after years of policy drift and transatlantic disagreement, the US and its allies will enter the talks in a position of relative strength and unity.

Consider the view from Tehran. It is on the defensive – caught red-handed in another nuclear deception. In contrast to the rancorous run-up to the war in Iraq, America and Europe are increasingly reading from the same script and Russia is signalling an openness to further sanctions.

The walkout of dozens of delegates during Mahmoud Ahmadi-Nejad’s anti-Semitic speech at the UN last week highlights Iran’s diplomatic isolation. Deciding the reputational and political risks are too great, many international banks and oil-trading companies have withdrawn voluntarily from Iran, which must import a third of its refined petroleum.

Domestically, Iran’s economy has been devastated by mismanagement, corruption, lower oil prices and fallout from the world financial crisis. Banks and foreign currency reserves are in rapid decline. Iran’s oil trust fund, which should hold tens of billions of dollars, has run dry. The bloody repression in Iran’s streets since June has compounded the damage. Iran’s ruling elite is divided, and the regime’s legitimacy is openly challenged – internally – as never before.

For years, the regime counted on bombastic language from Washington to distract its public from problems at home. No longer. Today there is no obscuring the fact that Iran is choosing repression over democracy.

For our diplomacy to have any success, two things are vital.

First, if Iran is not willing to negotiate in good faith, it must understand the consequences. Pressure is not an alternative to engagement; the two strategies complement each other.

UN Security Council sanctions are the most potent pressure, but there are also other levers. Insurance companies could be prohibited from insuring the Iranian tanker fleet. Export credit guarantees for Iran could be ended. Travel bans on human rights abusers could be enacted, Iranian assets seized, arms sales curtailed and investment bans enacted. Neighbours could cancel plans for natural gas pipelines linking Iran to the region’s energy distribution architecture. Some have proposed unilateral sanctions against foreign companies. While the prospect of such sanctions may goad other countries to action, we need to ensure unilateral efforts do not undermine the prospects of tougher international action.

Second, we must be willing to take yes for an answer. An important lesson of Iraq is that intrusive inspections can work. Our ability to detect and monitor the Qom enrichment facility for years before publicly revealing it is encouraging. One objective should be a more expansive inspections and monitoring regime to prevent Iran from diverting nuclear material to a “break-out” military programme.

While diplomacy with Iran was never going to be easy, the summer’s unrest has only increased the difficulty. In agreeing to talks, Iran has expressed an unexpected interest in discussing democracy and human rights. This is a conversation America should welcome, and an opportunity to demonstrate to the Iranian people that progress on the nuclear issue will not come at their expense.

Engagement may well fail. Ayatollah Ali Khamenei’s deep distrust of the US is no secret. The abuses of this summer may be the regime’s curt answer to US president Barack Obama’s outstretched hand. Given the turmoil, Iran may not even be capable of undertaking a sustained, strategic dialogue with the outside world.

And yet, it remains vital to seek a diplomatic solution to the stand-off. The international community is finally in a position to force Iran to choose either pariah status or a more constructive relationship with America and the world. Certainly the real possibility of either military conflict or a nuclear-armed Iran compels us to give diplomacy a chance.

The writer is chairman of the US Senate foreign relations committee

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