The call by the United States for the rest of Nato to take up more of the burden in Afghanistan is reasonable but unrealistic. Condoleezza Rice, the US Secretary of State, yesterday in Britain repeated the demand made tetchily last week by Robert Gates, Secretary of Defence.

But while US frustration with its Nato partners is understandable, it is foolish to think that such rebukes can overturn the strength of public feeling in Germany, for example. It would also be wrong to conclude from Nato's difficulties in Afghanistan that there is a terminal weakness in the alliance. Afghanistan was always going to be a difficult campaign, presenting enormous military and social challenges.

“Nato has got into a position where people talk about whether this long-term mission is an existential test,” argues Dana Allin, transatlantic specialist at the International Institute for Strategic Studies in London. “It's probably not a good idea to talk like that about a difficult mission which, while it would be a tragedy if it didn't work out, would not [then] pose an existential threat to Nato members.”

The scale of the task was driven home by new figures showing that while the opium crop for 2008 may shrink very slightly from the previous year's record levels, marijuana production will be at an all-time high. The annual survey by the UN Office on Drugs and Crime found that 192,000 hectares were planted for this year's crop, about half a per cent drop on the previous year — which had shown a record leap of a third on 2006.

Rangeen Dadfar Spanta, the Foreign Minister, said Afghanistan was determined to slash poppy production by 25 per cent this year. “We have only one choice,” he said. “Poppy can destroy us or we destroy the poppy.”

Britain, which has 7,800 troops, mainly in the south, has added its voice to the US call for more Nato forces and for governments to lift curbs on their troops' exposure to the fiercest fighting. Although Gates has taken particular aim at Germany for restricting its deployment to the comparatively calm north, that has received a firm rebuff. Germany said yesterday that it would send 200 combat soliders to northern Afghanistan but would not move any to the south.

The resentment of Britain and the US, which has 29,000 military personnel in Afghanistan, half attached to Nato, is understandable. Countries that consider themselves major players in Nato such as Germany, France and Italy “need to assume a greater share of the burden, including the combat burden”, the International Crisis Group (ICG) said. Stephen Harper, the Canadian Prime Minister, has said that he will bring Canada's troops home in February 2009 unless Nato allies step up their support, and may face a parliamentary challenge on the deployment next week. But for all the understandable frustration, the demands, if they are not simply brinksmanship, fail to recognise the political constraints on Nato members. A large majority of Germans - 85 per cent - are strongly opposed to sending their forces to the south of Afghanistan.

As Allin points out, Nato went into Afghanistan under its Article 5 - a commitment to stand by a member when attacked, as the US had been on 9/11. “It was necessary to do that or Nato would have been in a mess”. But the invasion has become a long-term mission directed to trying to help one of the poorest countries in the world. The problem is not with the strength of Nato but the evolving demands of the mission.

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