"We are in a combat zone the moment we leave this base," an American colonel told me at Bagram airbase, near Kabul. "We are shot at every day, several times a day." When I said that surely he had come to liberate and protect the people, he belly-laughed.

American troops are rarely seen in Afghanistan's towns. They escort US officials at high speed in armoured vans with blackened windows and military vehicles, mounted with machine-guns, in front and behind. Even the vast Bagram base was considered too insecure for the defence secretary, Donald Rumsfeld, during his recent, fleeting visit. So nervous are the Americans that a few weeks ago they "accidentally" shot dead four government soldiers in the centre of Kabul, igniting the second major street protest against their presence in a week.

On the day I left Kabul, a car bomb exploded on the road to the airport, killing four German soldiers, members of the international security force Isaf. The Germans' bus was lifted into the air; human flesh lay on the roadside. When British soldiers arrived to "seal off" the area, they were watched by a silent crowd, squinting into the heat and dust, across a divide as wide as that which separated British troops from Afghans in the 19th century, and the French from Algerians and Americans from Vietnamese.

In Iraq, scene of the second "great victory", there are two open secrets. The first is that the "terrorists" now besieging the American occupation force represent an armed resistance that is almost certainly supported by the majority of Iraqis who, contrary to pre-war propaganda, opposed their enforced "liberation" (see Jonathan Steele's investigation, 19 March 2003, www.guardian.co.uk). The second secret is that there is emerging evidence of the true scale of the Anglo-American killing, pointing to the bloodbath Bush and Blair have always denied.

Comparisons with Vietnam have been made so often over the years that I hesitate to draw another. However, the similarities are striking: for example, the return of expressions such as "sucked into a quagmire". This suggests, once again, that the Americans are victims, not invaders: the approved Hollywood version when a rapacious adventure goes wrong. Since Saddam Hussein's statue was toppled almost three months ago, more Americans have been killed than during the war. Ten have been killed and 25 wounded in classic guerrilla attacks on roadblocks and checkpoints which may number as many as a dozen a day.

The Americans call the guerrillas "Saddam loyalists" and "Ba'athist fighters", in the same way they used to dismiss the Vietnamese as "communists". Recently, in Falluja, in the Sunni heartland of Iraq, it was clearly not the presence of Ba'athists or Saddamists, but the brutal behaviour of the occupiers, who fired point-blank at a crowd, that inspired the resistance. The American tanks gunning down a family of shepherds is reminiscent of the gunning down of a shepherd, his family and sheep by "coalition" aircraft in a "no-fly zone" four years ago, whose aftermath I filmed and which evoked, for me, the murderous games American aircraft used to play in Vietnam, gunning down farmers in their fields, children on their buffaloes.

On 12 June, a large American force attacked a "terrorist base" north of Baghdad and left more than 100 dead, according to a US spokesman. The term "terrorist" is important, because it implies that the likes of al-Qaeda are attacking the liberators, and so the connection between Iraq and 11 September is made, which in pre-war propaganda was never made.

More than 400 prisoners were taken in this operation. The majority have reportedly joined thousands of Iraqis in a "holding facility" at Baghdad airport: a concentration camp along the lines of Bagram, from where people are shipped to Guantanamo Bay. In Afghanistan, the Americans pick up taxi drivers and send them into oblivion, via Bagram. Like Pinochet's boys in Chile, they are making their perceived enemies "disappear".

In Afghanistan, there has been similar carnage. In May last year, Jonathan Steele extrapolated all the available field evidence of the human cost of the US bombing and concluded that as many as 20,000 Afghans may have lost their lives as an indirect consequence of the bombing, many of them drought victims denied relief.

This "hidden" effect is hardly new. A recent study at Columbia University in New York has found that the spraying of Agent Orange and other herbicides on Vietnam was up to four times as great as previously estimated. Agent Orange contained dioxin, one of the deadliest poisons known. In what they first called Operation Hades, then changed to the friendlier Operation Ranch Hand, the Americans in Vietnam destroyed, in some 10,000 "missions" to spray Agent Orange, almost half the forests of southern Vietnam, and countless human lives. It was the most insidious and perhaps the most devastating use of a chemical weapon of mass destruction ever. Today, Vietnamese children continue to be born with a range of deformities, or they are stillborn, or the foetuses are aborted.

The use of uranium-tipped munitions evokes the catastrophe of Agent Orange. In the first Gulf war in 1991, the Americans and British used 350 tonnes of depleted uranium. According to the United Kingdom Atomic Energy Authority, quoting an international study, 50 tonnes of DU, if inhaled or ingested, would cause 500,000 deaths. Most of the victims are civilians in southern Iraq. It is estimated that 2,000 tonnes were used during the latest attack.

In a remarkable series of reports for the Christian Science Monitor, the investigative reporter Scott Peterson has described radiated bullets in the streets of Baghdad and radiation-contaminated tanks, where children play without warning. Belatedly, a few signs in Arabic have appeared: "Danger - Get away from this area". At the same time, in Afghanistan, the Uranium Medical Research Centre, based in Canada, has made two field studies, with the results described as "shocking". "Without exception," it reported, "at every bomb site investigated, people are ill. A significant portion of the civilian population presents symptoms consistent with internal contamination by uranium."

An official map distributed to non-government agencies in Iraq shows that the American and British military have plastered urban areas with cluster bombs, many of which will have failed to detonate on impact. These usually lie unnoticed until children pick them up, then they explode.

In the centre of Kabul, I found two ragged notices warning people that the rubble of their homes, and streets, contained unexploded cluster bombs "made in USA". Who reads them? Small children? The day I watched children skipping through what might have been an urban minefield, I saw Tony Blair on CNN in the lobby of my hotel. He was in Iraq, in Basra, lifting a child into his arms, in a school that had been painted for his visit, and where lunch had been prepared in his honour, in a city where basic services such as education, food and water remain a shambles under the British occupation.

It was in Basra three years ago that I filmed hundreds of children ill and dying because they had been denied cancer treatment equipment and drugs under an embargo enforced with enthusiasm by Tony Blair. Now here he was - shirt open, with that fixed grin, a man of the troops if not of the people - lifting a toddler into his arms for the cameras.

When I returned to London, I read "After Lunch", by Harold Pinter, from a new collection of his called War (Faber & Faber).

And after noon the well-dressed creatures come

To sniff among the dead

And have their lunch

And all the many well-dressed creatures pluck

The swollen avocados from the dust

And stir the minestrone with stray bones

And after lunch

They loll and lounge about