by Tom Coghlan

Thousands of US Marines stormed into an Afghan river valley by helicopter and land early today, launching the first major military offensive of Barack Obama's presidency with an assault deep into Taleban-held territory.

Operation Khanjar, which the Marines call simply "the decisive op", is intended to seize virtually the entire lower Helmand River valley, a heartland of the Taleban insurgency and the world's biggest heroin producing region.

It is the biggest operation launched by the US Marines Corps since the retaking of Fallujah in 2004 and seeks to break the grinding stalemate between Nato forces and the Taleban in the province.

US commanders stressed this morning their desire to move quickly and decisively with overwhelming force to seize the entire southern Helmand River valley from Taleban control ahead of the delayed Afghan Presidential elections on August 20.

"Where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces," Marine Corps Brigadier General Larry Nicholson, commander of the Marines in southern Afghanistan said in a statement.

He told his staff before the operation: "The intent is to go big, go strong and go fast, and by doing so we are going to save lives on both sides."

The 4,000 men from US 2nd Marine Expeditionary Brigade enjoy the support of their own integrated air wing, giving them more air support than the entire 8,000 strong British force has had at its disposal.

The US force went into action with the support of 650 Afghan troops, an operation by foreign ground troops on a scale unseen in Afghanistan since the Soviet withdrawal in 1989.

The operation would have an initial highly aggressive stage lasting 36 hours, AFP reported.

This morning wave upon wave of helicopters landed Marines in darkness at locations throughout the fertile river valley, a crescent of opium and wheat fields criss-crossed by canals and dotted with mud-brick homes.

Meanwhile hundreds more Marines raced by ground in convoys through the barren desert region that abuts the irrigated areas of the province. Known in the local Pashto language as the ‘Desert of Death', temperatures reach 50C at this time of year.

Captain Bill Pelletier, a spokesman for the Marines, said no clashes with Taleban troops had been reported, but a Marine had been slightly injured by a roadside bomb.

The operation was aimed at putting pressure on insurgents "and to show our commitment to the Afghan people that when we come in we are going to stay long enough to set up their own institutions," Captain Pelletier said.

He said the US military was prepared for casualties, but stressed that "it is absolutely essential that no civilians be harmed".

"We do not want people of Helmand province to see us as an enemy, we want to protect them from the enemy," he said.

The Marines hope by appearing suddenly and in overwhelming numbers, they can capture some of the Taleban's firmest strongholds with little resistance.

"Towns that were the Taleban heartland will fall. They will fall quickly. And hopefully they will fall without a shot. That's our intent," Brigadier General Nicholson said.

However, the greater challenge will be holding and stabilising such gains against Taleban re-infiltration and convincing a highly sceptical local population that Western forces will offer long-term security and improvements to their lives. The developing symbiotic relationship between the Taleban and Helmand's drugs mafia will further complicate that process. The province is the largest producer of opium in the world, the raw base for 90 per cent of the heroin used by British drug addicts. Drugs money has become a major source of Taleban funding and hundreds of thousands of local people are involved in the production and harvesting of opium poppy.

The insurgents have proved adept at reinfiltrating behind Western forces using the local civilian populace as cover. It is a problem that has beset the 8,000 British troops who have been thinly spread across Helmand, the country's largest province and roughly akin to twice the size of Wales, since they were deployed to the region in 2006.

Though Britain has doubled its troop presence since an initial deployment in 2006, they have been too few in number to do more than take and hold a few key islands of territory in the province.

Nato internal documents seen by the Times concede that 5 of the 13 districts of the province currently have no Afghan government presence at all. Ahead of the August elections Western forces are likely to attempt to cut the Taleban's supply lines southward, across the border to safe havens in Pakistan, whilst deluging the highly populated central parts of the province with Western troops.

The 10,000 Marines in Helmand Province, 8,500 of whom arrived in the last two months, form the biggest wave of an escalation ordered by President Obama.

He has declared the Taleban insurgency in Afghanistan and neighbouring Pakistan to be America's main foreign threat. Insurgent attacks in Afghanistan are at their highest since the militants were toppled in 2001.

Under President Obama the US force in Afghanistan is more than doubling this year, from 32,000 at the start of 2009 to an anticipated 68,000 troops by the end of the year, many of them diverted from Iraq. Other Western countries have about 33,000 troops in Afghanistan.

Addressing Marine commanders days before the assault, Dutch Major-General Mart de Kruif compared it to the D-Day invasion that changed the course of World War Two.

"We have people out there who do not realise that progress is about to come to them," he said. "We have enemies out there who do not yet realise that they are going to lose."

The governor of Helmand province, Gulab Mangal predicted the operation would be "very effective".

"The security forces will build bases to provide security for the local people so that they can carry out every activity with this favourable background and take their lives forward in peace," the governor said in a Pentagon news release.

Local phone masts in the area affected by the operation appeared to have been switched off this morning, though it was not clear whether this was due to US or Taleban action.