Shortly before he left Iran for New York, Mahmoud Ahmadinejad attended a military parade featuring army lorries emblazoned with the slogans “Death to America” and “Death to Israel”. The Iranian President also told a US interviewer who met him in his garden in Tehran: “I cannot tell a lie.” Less than 24 hours after the interview was broadcast he was telling an audience at Columbia University that “we do not have homosexuals like in your country” and that Iranian women, who in recent months have been detained in large numbers for failing to cover themselves in public, were “the freest in the world”.

An advantage of autocracy, from the point of view of the autocrat, is his freedom to assert his honesty and not care whether or not he is believed. A far greater advantage of free speech is the ability to test such assertions against real evidence. On his theatrical – and ultimately illuminating – visit to America, Mr Ahmadinejad has provided evidence of nothing so much as sheer mendacity. It is to be hoped that his audience at the UN last night bore this firmly in mind as he lectured them, once again, on Iran’s “peaceful” nuclear intentions.

Mr Ahmadinejad is no more truthful on his nuclear plans than on homosexuality, women’s rights or the Holocaust, which he has called a myth. Even his claim to have a right to develop peaceful nuclear reactors is false: under the terms of the Nuclear Non-Proliferation Treaty, which Iran has signed, it has forfeited that right by building the plant for highly enriched uranium production that has triggered two sets of UN sanctions in the past year. Neither set has had any significant effect. Iran has refused to cease its enrichment programme or accept the “intrusive inspections” that the International Atomic Energy Agency (IAEA) is required to carry out in the circumstances.

Iran has no compelling economic need for civilian nuclear reactors. In a perfect world this would not constitute an argument against building them, but power generation is not the purpose of the cascade of 3,000 uranium centrifuges being built at Natanz. Acquiring nuclear weapons is a central aim of Mr Ahmadinejad’s blinkered brand of Persian nationalism, which aims to assert influence far beyond Iran’s borders.

When President Sarkozy of France called a nuclear-armed Iran “an unacceptable risk for regional and world stability”, he was merely articulating common sense. It is not too late to avoid that risk through diplomacy, but time is running out. France, the US and the UK have called for a third UN Security Council resolution and a new set of sanctions to punish Tehran for persistently ignoring its obligations under the NonProliferation Treaty. Such a resolution is unlikely in the short term, largely because of an unhelpful deal struck with Iran by the IAEA on a miscellany of peripheral nuclear-related questions, giving Russia and China a pretext to continue to drag their feet over joining Europe and the US on the core issues of inspection and suspension of enrichment. But even in the absence of new UN sanctions, the EU could usefully tighten its own.

Mr Ahmadinejad has shown why the world should be more concerned about his intentions for Iran. He has condemned himself in his own words.