Sitting cross-legged on his hospital bed, Mohammed Hussein Nejad still drapes around his neck the chequered black-and-white scarf he wore as a young soldier during the war between Iran and Iraq. It is a symbol of the sacrifice he made for his country and for Islam. Wheezing, he puffs on his inhaler.
"Since 1987, I have this problem because of the chemicals," he says. "I have this burning sensation, and I've been to hospital seven times."
Mohammed will never forget the day in 1987 when he looked up from his position on the Faw peninsula in southern Iraq and saw three Iraqi fighters overhead. Each dropped bombs that exploded in the air, distributing a white gas.
"One of my comrades was hit by shrapnel and I went to rescue him. It was too late to get my mask because I had left it in the vehicle. From then until today, I've had this lung infection."
Dr Hamid Sohrab-Pour, who treats patients like Mohammed at Tehran's Sassan Hospital, says some 100,000 Iranians - mainly troops, but also many civilians - were affected by Iraqi chemical attacks. Ten per cent died immediately and at least 5,000 are still undergoing treatment.
"There is an antidote for nerve gas, but it has to be used within 15 minutes," he says. "For mustard gas, there is no antidote and, all these years later, we can offer only symptomatic treatment. Those who died are lucky, because they do not suffer now."
No wonder, then, that Iranians hate and fear Saddam Hussein. But America's threats pose a dilemma for the leaders of the Islamic republic: they do not want to see their other enemy, the Great Satan, expand its influence in the region.
"How can we turn to a python to repel the menace of a scorpion?" asked Hashemi Rafsanjani, a leading cleric and former president. America's support for Iraq during the eight-year war has not been forgotten, and while Iran's official position on America's proposed assault is "active neutrality", the possible consequences of US action - an influx of refugees, a drop in the oil price, instability on the borders - may demand an Iranian response.
Iran is already involved because it provides sanctuary to one of the main Iraqi opposition groups, the Supreme Council for Islamic Resistance in Iraq (SCIRI) and its armed wing, the Badr Corps. SCIRI's leader, Ayatollah Baqir al-Hakim, is a Shi'a Muslim like the Iranians. Saddam Hussein's regime is dominated by minority Sunni Muslims, and al-Hakim fled in 1980 after he was imprisoned and tortured. His fighters are Iraqi Shi'as who defected to Iran during the Iran-Iraq war.
Today, uniformed soldiers guard SCIRI headquarters in downtown Tehran, and visitors are ushered into a room where al-Hakim - with his grey beard and white turban - sits under portraits of more than 20 family members killed by the Iraqi regime. After the Gulf war in 1991, the Shi'as rebelled against the Iraqi leader, but the Americans refused to back the uprising, partly because they feared that al-Hakim would install a revolutionary Islamic regime modelled on Iran.
Al-Hakim's emissaries have been in Washington to co-ordinate with the Americans a softening of the Islamic revolutionary line. Iran's die-hard clerics who are trying to hold back a tide of reform initiated by the country's popular president, Mohammad Khatami, are worried. At Friday prayers, the conservative Ayatollah Mohammad Yazdi links the threats against Iraq with America's "hidden plot" to westernise Iran by corrupting the young. "Behind the scene of American warmongering and its unilateral moves lies the aim to prevent Islam from growing," he thunders to a crowd that responds with ritual cries of "Death to America".
Iran's reformists, led by President Khatami, would like to use the Iraq issue as a way of opening contact with America, but their experiences after 11 September make them wary. Initially, Iranians demonstrated in the streets to show their sympathy with American victims, then the government allowed western aid to go across Iran's borders into Afghanistan, and now it has captured and turned over to western and Arab countries more than 20 al-Qaeda suspects. Iran's reward was President Bush's January speech, branding the country, along with Iraq and North Korea, as the "axis of evil" because of its suspected nuclear weapons programme and continued support for the anti-Israeli Hezbollah force in Lebanon.
"The wall of mistrust between the US and Iran is very thick," says Kamal Kharrazi, the Iranian foreign minister and a leading reformer. "It is thicker after Afghanistan. There were golden opportunities which the Americans missed."
The conservatives fear that an attack on Iraq would presage an attack on Iran. If they feel trapped, they may clamp down on the government's moves towards democracy and reform.
For those who cannot forget the war that robbed Iran of 300,000 young men, the calculations are different. At the Martyrs Cemetery in Isfahan, an old woman dressed in the all-concealing black chador kneels weeping at a grave. A black-and-white photograph of a young man stares out from the headstone.
"Saddam Hussein killed my son," she says.
"Would you like to see him fall from power?"
"It's my only wish."
"Would you like the Americans to kill him?"
"Yes," she replies, pulling her black cloak around her.
Lindsey Hilsum is Channel 4 News diplomatic correspondent