The transfer yesterday of American troops in eastern Afghanistan to Nato’s International Security Assistance Force (Isaf) brings under British command the alliance’s biggest ground operation in its history. It also marks the biggest deployment of US forces under foreign command since the Second World War. Some 12,000 US troops were placed at the disposal of General David Richards, joining the British, Dutch and Canadian forces fighting the resurgent Taleban in the south.

And at the formal transfer ceremony General Richards underlined the Nato resolve to use the new streamlined command to enhance the effectiveness of all the foreign forces in Afghanistan, now numbering 31,000 troops.

It is five years, almost to the day, since the US-led coalition launched its attack on Afghanistan, then under the repressive rule of the Taleban and a training ground and haven for al-Qaeda. Since then, the military, political and economic convulsions have transformed the country, bringing an elected government to Kabul, liberating Afghanistan’s women from the serfdom to which the obscurantist regime had condemned them and offering the country a chance to rebuild and reawaken.

Nevertheless, the Taleban are still far from defeated. They now pose the greatest threat to the country since they were ousted from power. Attacks on schools and government installations have grown, suicide bombers have hit allied convoys and more than 140 foreign troops have been killed since January.

It is clearly time for a strategic re- assessment. The Taleban offensive has begun to take a political toll. Public opinion in much of Europe, already alienated by the enduring strife in Iraq, is wavering. Several governments have refused Nato’s urgent calls for more troops to reinforce the overstretched forces in the south. There is needless talk of an unwinnable war and the lessons of history.

Such talk is irresponsible and self- defeating. There is no inevitability of a coalition defeat; indeed, General Richards pointed to the substantial damage done to Taleban forces by Nato’s two-week offensive last month, when around 1,000 insurgents were killed. The Afghan National Army is proving increasingly adept and effective. Some 37 nations have sent troops to Afghanistan to bolster the Karzai Government, and Nato’s determination to take military operations deep into the areas dominated by the disaffected Pashtun and Taleban warlords is an effective response to his pleadings for more allied help beyond Kabul.

Winter is coming on. By the end of this month, snow will have covered most of the peaks in the mountainous east — though not in the warmer Helmand Valley. Passes will be blocked and the Taleban will find it harder to mount attacks. Nato should make the most of this opportunity. The Taleban will regroup and attempt to re-equip with arms smuggled in from Pakistan. Nato, better equipped, must allow no let-up, must interdict the supply routes and prepare the offensives to be launched while the Taleban are weak.

Coalition forces, meanwhile, should show a more visible presence in the villages, where they can give backing to reconstruction and bolster support for the Karzai Government. Doom-mongering is misplaced. More British help for injured soldiers will bolster public support here. But all should understand that saving Afghanistan is a moral and military imperative.