His remarks about wiping Israel "from the map" have prompted calls for Iran to be thrown out of the UN and led to dark hints about possible military action. Even the EU is no longer giving the regime the benefit of the doubt over its suspect nuclear weapons programme.
ATTITUDE TO ISRAEL
As a former member of the fanatical Revolutionary Guards, Mahmoud Ahmadinejad must have been chanting this slogan for years. But now he is president his words have a quite different impact: hence Tony Blair's incandescent reaction on Thursday and the international outrage.
Western leaders do not believe Mr Ahmadinejad is launching a new campaign against Israel, which Iran does not recognise, but the ideology of the Islamic republic is now exposed. A public presidential comment such as "wipe Israel from the map" has changed the diplomatic environment at a stroke and risks further isolating Iran.
Fears that Iran is pursuing a nuclear weapons programme were heightened in August when the regime carried out a threat to resume uranium conversion - the possible first step towards eventually building a nuclear weapon - causing the collapse of negotiations with Britain, France and Germany.
Mr Ahmadinejad's defiant speech to the UN summit last month did further damage. Israel, the US and the EU suspect Iran is attempting to build a bomb under cover of a civilian nuclear energy programme. The International Atomic Energy Agency says there is no evidence of a weapons programme, but is calling on Iran to provide a full account of its nuclear-related activities. Iran faces referral to the UN Security Council for possible sanctions - as early next month - unless it comes clean.
INTERFERENCE IN IRAQ
Britain has presented what it describes as evidence that Iran was behind frequent roadside bomb attacks on British troops in southern Iraq. Britain links the improvised devices to the Palestinian Islamic militants of Hizbollah and their sponsor Iran.
British officials have also accused the Revolutionary Guard, which has close links to Tehran, of teaching Shia fighters to make the bombs in Iraq. The US has warned Iran against stirring up the insurgency in Iraq.
STATE SPONSORSHIP OF TERROR
Iran has long been accused of state sponsorship of terror over its links to the militant Palestinian groups Hamas, Hizbollah and the Islamic Jihad which have all had offices in Tehran.
Mr Ahmadinejad praised Palestinian suicide bombers this week on the day Islamic Jihad claimed responsibility for a bombing in the Israeli town of Hadera that killed five people. The US and Britain say Iran provides the groups with funding, training and weapons.
WHO'S IN CHARGE?
The supreme leader in Iran is Ayatollah Khameini and he has the final say in all matters of state: until August, he had backed negotiations with the EU which could have sealed the bargain in which Iran gave up its nuclear ambitions in return for technological and economic benefits. It remains to be seen whether Mr Ahmadinejad will be marginalised as power fractures inside the country.
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