They were not hard to spot -- the dead tanks -- as they littered the sides of the main Baghdad-Tehran highway deep inside Iran. Heavy twisted monsters, blasted by artillery, mounted on stone plinths like trophies as a warning to any other army that came to fight and die here, as Saddam's divisions had done. After 40 I stopped counting.

On the Iranian border itself the little town of Mehran had become a shrine to martyrdom and death. Like a mini-Stalingrad, it had been razed three times during the Iran-Iraq war of 1980-88, its streets filled with the corpses of Iranian child soldiers sacrificed in human-wave assaults; but in the end the Iranians expelled the invader at an awesome human cost.

Saddam has gone, but Mehran is once more in the front line of potential war. The Iran-Iraq border is just a few miles to the west of the town on a flat plain -- ideal tank country. The border itself is marked by a meandering stream but on either side now are the opposing armies of the United States and the Islamic Republic of Iran, all waiting for orders from above. If the Americans do ever invade then it will be here, as the shortest distance to Tehran from Baghdad; and that little stream the Rubicon for a war of unimaginable consequences.

In No10 the tom-toms of war of war are drumming again as Tony Blair warns that he will not tolerate the meddling hand of Iran in the affairs of Iraq. In Washington the neoconservative tom-toms are even louder, warning that the West must "surgically strike" at Iran's hidden nuclear facilities and robustly challenge Iranian state-sponsored terrorism. Nor it seems can the EU countenance Iran's rise as a nuclear power either. A new nuclear crisis now looms later this month with the threat of UN Security Council sanctions over Iran's controversial nuclear programme.

In Tehran the hardline President, Mahmoud Ahmadinejad, has done little for foreign relations with his chilling call for Israel to be wiped off the map. We are, it seems, close to the on-ramp for another spectacular confrontation in the Middle East.

But before we succumb again to the hysterical warnings of our leaders it is worth seeking a cold-eyed measure of this new enemy they would have us fight. Iraq and Iran are very different. Iran is nearly four times the size of its neighbour and six times the size of Britain. How could an already undermanned American army expect to control such a huge territory?

Nor will those already fabled "surgical strikes" by the US Air Force deliver a decisive blow to Iran's growing nuclear capability. Iran's nuclear plants are already well hidden across its huge land mass. And all that a partial strike will do is unleash an unstoppable war without significantly damaging the enemy's capability.

Iran's population at 70 million is three times that of Iraq's and it has one of the youngest populations in the world. Iran's standing army is estimated by the CIA to be 520,000-strong, but each year 817,000 17-year-old Iranian boys are potentially available for military service. That is an awful lot of martyrs or suicide bombers.

The Iranians are Persians, not Arabs, a consideration entirely absent from most neoconservative analyses of Iran's supposed weakness. Persian imperial dynasties date back to Cyrus the Great, around 530BC, and Xerxes, 486-465BC, who plagued the Greeks.Unlike the chaotic Arab shambles of Saddam's Iraq, Iran remains a hierarchical society where the vast majority live in rigid terror of the authorities above them, religious or imperial, and will utterly obey their commands.

In many ways Ayatollah Khomeini, who came to power in 1979, was the greatest Persian Emperor, fusing his own version of Shia Islam into a state ideology. And during the Iran-Iraq war he revived the ancient Shia tradition of martyrdom: hundreds of thousands of soldiers, many of them children, died in futile suicidal assaults over minefields. "The Tree of Islam has to be watered with the blood of martyrs," said Khomeini without regret.

Martyrdom is still the state religion. Huge posters of the war dead and Palestinian and Lebanese suicide bombers dominate every surface in Tehran and every speech of the political leadership. Any attempt to threaten or invade Iran will be a huge asset to a regime longing to re-energise its faded legitimacy among its own downtrodden population. Invasion by the Great Satan would be a godsend.

Nor should we underestimate Iran's capacity to punish its enemies at long range. In 1982 Iran sent a thousand revolutionary guards to Lebanon to spread the Islamic revolution. The plan failed but Iran was behind three of the greatest acts of postwar terrorism: the American Embassy bombing in Beirut and the blowing up of the US Marine and French paratrooper barracks by suicide bombers in 1983. The French and the Americans left Lebanon in defeat soon afterwards.

Iraq is a mess but widening the conflict by attacking Iran would be an act of madness. That little stream on the western edge of Mehran is a Rubicon we must never cross.

Kevin Toolis is a terrorism expert and a documentary film-maker