The UK is committing 1,400 more troops to Afghanistan, defence secretary Des Browne told the House of Commons today.
The "challenging" security situation in the south and east of the country demands additional troops, Mr Browne told MPs, adding that it is also important to defend and reinforce the past five years of "progress".
"Before we talk about what more we must do, we must understand what is at risk if we do not continue to live up to the collective commitment we have made to Afghanistan and its people", he told MPs, before detailing the schools, mosques, roads and wells built across the country.
However, Mr Browne admitted that the UK has been unable to persuade its Nato allies to share the additional responsibility in Afghanistan.
He said: "We believe that every Nato partner should be prepared to do more to meet this need. I have lobbied our partners consistently for more help in these regions and I will continue to do so.
"But it is increasingly clear that at present, when it comes to the most demanding tasks in the more challenging parts of Afghanistan, only we and a small number of key allies are prepared to step forward," he said.
The majority of the troops will deploy over the summer and be based mainly in Helmand province, and also in Kandahar airfield.
The additional troops will increase the total number of personnel from 6,300 to 7,700 and Mr Browne confirmed that they were committed until 2009.
He firmly rebuked speculation that last week's announcement of troop withdrawals from Iraq was ordered to allow the additional deployment to Afghanistan.
"The situation in Iraq determines what we do there, not the situation in Afghanistan," the defence minister insisted.
Last week, the Liberal Democrats welcomed the initial reports of additional deployments to Afghanistan, but warned that the army must not be overstretched.
Defence spokesman Nick Harvey argued: "Any hope of maintaining our commitments to Afghanistan relies on us being able to get our troops out of Iraq.
"There must be clear objectives and clear aims, rather than the confusion that has surrounded much of the mission."
He added: "It is vital that pressure is kept up on other Nato countries to ensure that they pull their weight. Failure would be disastrous both for Afghanistan and the alliance."
Conservative defence secretary Liam Fox also questioned the failure of some Nato members to match the UK's commitment.