President Karzai will have a tough job to persuade those he meets in London this week that his Government can meet the challenges in Afghanistan.

The Taleban insurgency is growing in strength and tactical sophisticated. Allied casualty figures are rising as vital supply lines to the capital come under siege. Suicide bombings have become more frequent and more daring. The Government is bogged down in bickering, its authority draining away as top officials are tainted with corruption charges. Disillusion has set in among commanders and diplomats, and some voice the conviction that, as the war is now being conducted, victory looks impossible.

A fundamental reassessment of the Afghan conflict is now under way. Three factors have accelerated that rethinking. The first is the appointment of General David Petraeus, the architect of the successful surge strategy in Iraq, as head of the US Central Command with responsibility for Afghanistan. The second is the increasing anger of Nato members on the front line with those countries that have “caveats” over sending their troops into battle. And the third overwhelming factor is the election of Barack Obama, with a mandate to halt the drift into disaster in this war.

The Bush Administration is to recommend a rapid expansion of the Afghan Army, now numbering about 67,000, to a target of 134,000 in the next six years. This will cost an estimated $17 billion - a huge sum, but one far cheaper than trying to fill the gap by rotating even more Nato troops into provinces where the security situation is increasingly precarious. The strategy fits with the plan by General Petraeus to enlist tribal leaders as battlefield allies against the Taleban, much as he did in Iraq. The tactic may be resisted by President Karzai, who has tried to bolster his crumbling authority by favouring various corrupt warlords. It must be made clear forcefully to Kabul that this is not working.

The tribal structure is far more complicated in Afghanistan than in Iraq. Ancient rivalries and internal power struggles are often decided by paying tribesmen to change sides. Talks go on at the same time as fighting. There is a recognition that Kabul will have to negotiate with the Taleban. But any attempt to do so now would fail: the Taleban respect only military strength, and that, plainly, is what the allies lack.

In the short term, therefore, the Nato forces must be made far more effective. This means, first, that the present divided commands must be unified. Secondly, the numbers must rise, a point made forcibly by Mr Obama during his campaign. Another US brigade will join the 31,000 US troops in January, and more will arrive as the army in Iraq is withdrawn. The Europeans must match this increase. Thirdly, the rules of engagement must be standardised. If Germany has a constitutional ban on sending troops into combat, it must at least dispatch to Kabul the promised police trainers.

A new strategy will work only if the political framework improves. President Karzai is now proving an obstacle, undermined by corruption, cronyism and obstinacy. He should be thanked for his past leadership but warned that he cannot automatically count on Western support in future elections. It will be up to the Afghans to chose a president who can inspire greater trust.