It was one of the most daring, dangerous and spectacular operations of the war in Afghanistan.

Some 5,000 troops - 3,000 of them British - fought for six days through the heart of Taliban-controlled territory on a mission of utmost humanitarian importance.

By the end of the operation, 250 Taliban fighters had been killed yet only one British soldier was injured.

Such was the success of the project to restore a hydroelectric dam that the commander of British forces in Afghanistan has called it the 'end of the beginning' of the campaign against the Taliban.

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The operation was completed on Monday night but due to an official veil of secrecy only now can its events be told.

The aim was to install a new turbine at the Kajaki Dam in the centre of the country which could eventually provide power to 1.5million Afghans.

But because the 200-ton turbine comes in seven parts, each weighing up to 30 tons, and there is no suitable airstrip near Kajaki, the only way to transport it was by road from Kandahar airbase 100 miles away to the south-east.

The main route is by the 611 Highway but elite troops from the Parachute Regiment's Pathfinder Platoon discovered a mountain pass - codenamed Route Harriet - to allow the convoy to bypass it near the end.

The Paras tricked the Taliban by flying hundreds of troops by helicopter into Kajaki, as if they were expecting a convoy on the 611.

Convoy: Armoured vehicles wait to offload the massive turbine after an extraordinary 100-mile journey through Taliban territory

Then around 150 of the Paras, backed by 400 Afghan troops and their Royal Irish mentors, pushed south on the highway to probe the Taliban lines.

Lieutenant Colonel Huw Williams, the commanding officer of 3rd Battalion, the Parachute Regiment told his troops the night before:

'There are reports of more fighters coming into the area. That's more people to kill and I have got no problem with killing them.'

As the men left camp they came under a volley of enemy fire.

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The dam will eventually provide energy to much of Afghanistan

In reply, Apache helicopters launched Hellfire missiles and fired 30mm cannons, while warplanes dropped 500lb bombs and artillery smashed the Taliban positions.

Late that night - last Tuesday - the 200-vehicle strong main convoy left Kandahar airbase, with the turbine sections surrounded by layers of steel to make them look like ordinary containers.

U.S. and British special forces were dropped ahead of the convoy to sweep through villages along the treacherous Helmand river valley, 'sanitising' Taliban strongholds to allow the convoy safe passage.

Engineers said getting through dry rivers and heavily-mined mountain passes was the largest clearance operation undertaken by British forces since the Second World War.

Then, last Saturday, British soldiers tried to cut a deal - offering around £12,500 to local elders for further safe passage of the convoy.

The elders agreed and at first the deal went well, with reports showing insurgents had removed and destroyed roadside bombs. But by morning the deal was off.

'We took them the papers to sign and no one turned up,' said Captain Steve Boardman, who led the negotiations. 'I think the local Taliban agreed, but their highers in Pakistan said no.'

Afghan troops used VHF radios to harangue the Taliban across no-man's land.

When one of the Afghans asked why the Taliban wouldn't let the turbine through, the insurgent replied: 'We don't need electricity. We have Islam.'

Undeterred, three companies of Paras pushed through the Taliban stronghold of Kajaki Sofla, stopping only to destroy pockets of insurgent resistance.

The convoy finally arrived under cover of darkness at its destination late on Monday night and took more than six hours to unload.

Hailing the success of the operation, Brigadier Mark Carleton-Smith, the commander of British forces in Afghanistan, said:

'I would sense that in the sweep of the campaign, this marks the end of the beginning. We understand the problems here much more clearly.

'We have a strategy that looks as though it may work.'

Mine sweepers: Soldiers from Britain's 3rd Battalion the Parachute Regiment clear the road to Kajaki in preparation for the turbine to pass through the area