When Paddy Ashdown arrives in Kabul, probably in April, he will be expected to achieve nothing less than to save the faltering Western mission in Afghanistan.

It would have been a daunting challenge at any time in the past three years since the Taleban resurgence began, but with constantly worsening security and creeping demoralisation in Kabul it is now a taller order than ever.

There will be many well-wishers — surprisingly large numbers of disillusioned Afghans will see Lord Ashdown of Norton-sub-Hamdon as their last chance — but many spoilers as well, and not just from the Taleban side.

Whether he succeeds or fails may decide the outcome of the West's sometimes dysfunctional efforts in Afghanistan, and success will depend in part on how much power he has. Even more important may be the influence he can exert and the level of support he receives from Kabul's foreign backers.

His role will be to co-ordinate policy between the foreign governments, international agencies, Nato militaries and the Afghan Government. It will be far more significant than that of any previous UN envoy in Kabul. His relationship with the frail Afghan Government of President Hamid Karzai will be key.

Both Afghans and foreign diplomats in Kabul are hoping fervently that he will be able somehow to help to turn the ramshackle, incompetent and increasingly corrupt machine of state into a modern administration that can govern efficiently and with determination.

Lord Ashdown will also have to deal with the plethora of foreign militaries fighting a complex war, and in particular a powerful US military not known for listening to non-US citizens.

As a former soldier and a politician with a track record from the Balkans, Lord Ashdown was far better placed than any possible rival to take over the role dubbed “super envoy” in a country where conflict rages and which feels as if it could be slipping towards the edge of an historic disaster. His most important role will be co-ordinating the chaotic and disorganised aid and reconstruction effort pursued by dozens of different agencies and nations with widely differing policies. They have poured billions of dollars into Afghanistan but ordinary Afghans have seen little benefit and their grievances are both growing and fuelling insurgency.

For all the difficulties, Lord Ashdown will start with some important advantages. He met President Karzai in Kuwait a few weeks ago and the two reportedly got on well. But if that relationship turns sour there is potential for bitter conflict between the two.

President Karzai has developed a tendency to blame foreigners for many of his country's problems, including his own backers. Blaming foreigners plays well with an electorate growing increasingly frustrated with the failure of Western powers to implement their promises.

Former Norther Alliance warlords, who are politically powerful, may wish to see him fail.

There is also a danger of friction between Lord Ashdown and Nato generals. Air bombardments that have killed large numbers of Afghan civilians are deeply emotive and highly damaging to the West. A civilian may want the generals to change military strategy - but with so few troops available, the generals rely on air power. Lord Ashdown is expected to have no formal say over military decisions but may have to find a way to exert influence. The UN is distinctly nervous about the important but rather undefined relationship their new envoy will have with the military.

America, increasingly fearful of failure in Afghanistan, has backed Lord Ashdown's candidacy after realising slowly that a new approach is needed urgently .

Britain has not backed him with so much enthusiasm. Gordon Brown reportedly fears that if Lord Ashdown succeeds, the former Liberal Democrat leader will get the credit, but if he fails, Britain will get the blame.

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