It started with a routine prisoner search.

Taliban prisoners were lined up in the courtyard of the huge mud-walled Qala Jangi fortress that dominates the northern city of Mazar-e-Sharif with American special forces men and a few journalists curiously peering at them.

These prisoners had endured a two-week Northern Alliance siege of the neighbouring town of Kunduz. None of them were thought to have any fight left in them. But at least one of them did.

Accounts of precisely what happened differ according to who you talk to - and sometimes the same witness will give two separate accounts.

But while a group of a dozen or so were being searched for weapons and then bound with cord around their backs, a fight broke out. Some say a grenade went off. Some that a prisoner had a concealed weapon. Others that a prisoner simply took advantage of a lackadasical guard, snatching a machine gun and turning it on his captors.

Whatever happened, within seconds machine gun bullets cut down several guards, with the others, plus the Americans and the other guards, running for their lives.

The battle spread like a forest fire around this sprawling fortress, which extends over several acres.

Surviving soldiers ran for their lives. A Red Cross delegate jumped onto the roof of an outbuilding to escape.

It was the start of the worst firefight in this war so far and when it was over more than 400 prisoners, 40 guards and one CIA officer lay dead.

While even eyewitnesses disagree about how it started, and what were the motivations of this suicidal revolt by the prisoners, TV viewers across the planet know what happened next.

The Northern Alliance commander, the huge ,bearded Gen Rashid Dostum, called up reinforcements.

And the Americans, with one man down, piled on the fire power.

Jets screamed in to drop bombs at point blank range.

Artillery hammered hastily built strong-points along the walls. Prisoners trapped around a watch tower fired bazookas and mortars taken from an ammunition store room at the assault troops.

Finally, Dostum's men scaled the walls, and poured machine gun fire down onto the hordes of prisoners below.

One of Dostum's officers, Commander Alam, summed it all up: "They're fighting until death. They won't hand themselves over alive." One US bomb went off target, wounding five American special forces men.

Others were seen scurrying around in desert camouflage, shooting from the hip, far from the serene mysterious image they had cultivated until now.

And still the prisoners kept fighting. Troops arriving on Monday were met by gunfire from the basement of a red-brick former school building.

On Tuesday, having failed to persuade whoever it was in the basement to come out, huge drums of oil were tipped over a ventilation shaft, flooding the basement. Then the oil was set alight.

And on Wednesday someone down there wounded a further two soldiers, with more burning oil being poured in. Now the basement has been sealed, creating a tomb for anyone still down there.

Alliance sources say that "almost none" of the prisoners surrendered alive.

And this is the greatest mystery of all, certainly for Europeans used to the idea that surrendering soldiers are treated as prisoners of war.

But these soldiers were special. They were not Afghans, but Arabs, Pakistanis and handfuls of Uzbeks and Chechens, all of them loyal not to the Taliban but to the al-Qaeda network run by Osama bin Laden.

And this was the crucial difference. A difference explained to me a month before by a one-armed deputy commander with the First Mazar-e-Sharif brigade, Faziludin.

Like every man in his brigade, Faziludin had been lucky to escape with his life from Mazar when it fell, for the second time in two years, to Taliban forces in 1998. Now, sitting in a compound near the front line, waiting for the order to attack, he spelled it out.

"If we capture Mazar-e-Sharif, when I see Taliban who are Pakistanis or Arabs or Chechens, I will kill them. If they are Afghans, they will be spared. They are our brothers, you see." Now Faziludin has got his wish, though hardly in the way he intended.

Mazar did indeed fall, three weeks ago, but the Arabs, Chechens and Pakistanis based there bolted rather than put up a fight. All the way to Kunduz.

Most knew the fate that probably awaited them. It is the Afghan way of war that fellow Afghans who surrender are almost always taken prisoner and allowed to go home.

For a country perpetually at war, this has two important practical benefits. First, it encourages the losing side to give up fighting once the result is clear, making things easier all round. And secondly, it recognises the chance that today's captors might be tomorrow's prisoners.

But foreign forces have no such advantage. Whether they were British soldiers from the 19th century or Arab fundamentalists today, they were always viewed with contempt.

On top of this, the prisoners herded into Qala Jangi knew something else - the recent bloody history of Mazar-e-Sharif.

In 1997 the Taliban first took the city after Dostum was betrayed by one of his deputies General Abdul Malik, who opened his front lines to let the Taliban forces in.

But two days later Malik, incensed that the Taliban refused to give him command of Dostum's city, turned against them. More than 3,000 Taliban were killed, including, notoriously, 1,200 who were put into metal containers, dumped in the desert, and left to literally bake alive.

The following year the Taliban were back, and this time they took their revenge. Taliban units swept into Mazar, machine gunning pedestrians as they went and killing 6,000 - a record even by Afghan standards.

And now Dostum is back again, and nobody on either side has any illusions about what will follow.

The slaughter at Mazar has confirmed that this is a war far from over.

And it sets an ominous precedent for the kind of fanaticism the US Marines now massing outside the Taliban nerve centre of Kandahar may encounter.