The shock and revulsion at the brutal killing of five British soldiers by an Afghan policeman at a checkpoint in Helmand yesterday have thrust this bitter war all the more violently into the centre of debate. The murders bring the number of British soldiers who have died in Afghanistan since 2001 to 229. This is the worst year for military casualties since the Falklands conflict in 1982. The killings come hard on the heels of the aborted presidential elections, the graphic evidence of corruption within the Afghan Government and the call by Kim Howells, a former Foreign Office minister, for the withdrawal of most troops. As the Taleban calculated, this has undermined support for Britain’s engagement. Many people now believe the conflict unwinnable and Afghanistan a political and military disaster.

This is fast eroding confidence in the Government’s handling of an ugly war. It has revealed the Prime Minister as woefully unprepared to deal with the widespread public anxiety: unfocused on the military details, unclear on the strategy and unwilling to spell out the options and alternatives. Mr Brown seems to believe, with Panglossian naivety, that the conflict can be contained without sending any more troops; that President Karzai will root out corruption; and that the US will come to the rescue. There is little evidence that any of this will happen.

It is time for this Government to stop hoping for the best and start preparing for the worst. What if President Karzai ignores the demands from Washington to sack his corrupt cronies, curtail the culture of bribery, halt the lucrative opium trade and distance himself from the warlords and criminals enriching themselves on Western aid? What if the Taleban step up the roadside bombs and suicide attacks and kill hundreds more British soldiers? What if the infiltration of the police undermines the entire Afghanisation programme? And what if President Obama chooses not to send the 40,000 troops requested by his generals and opts for only half that figure?

Wise generals plan for any contingency; Mr Brown appears to avoid thinking ahead. His speech to the Labour Party conference was shocking for its scant attention to the war and hackneyed response to the growing doubts. Mr Howells said Britain would do better to fight the al-Qaeda threat on the streets of Britain. It is a serious argument that needs a serious response. What should be the appropriate troop levels and deployment of Britain’s forces in Afghanistan? What would be the repercussions of a pull-out?

It is not enough simply to say that Britain cannot just walk away from the conflict. More and more people are asking: why not? If the Government believes, rightly, that a precipitate withdrawal would destroy Nato’s credibility, galvanise the Taleban, render worthless the sacrifices already made and take Afghanistan back to the brutal rule it suffered before 2001, then the case must be made. Extremists around the world would gloat at their victory. Pakistan, brittle and fissiparous, would be threatened with an Islamist takeover. Al-Qaeda’s deceptive and blasphemous campaign would win new adherents — in Mumbai, Istanbul, Somalia or North Africa.

Britain needs to make clear, to its people and its forces, that it is the junior partner in this war, and that strategic decisions can be made only in lockstep with Washington. It must also set much tougher terms for dealing with President Karzai: if he refuses to punish corruption and sideline his brother, Britain should circumvent Kabul and focus its energy and investment on local governments. Everyone, in Afghanistan and the West, wants the foreign troops to leave. The question is how and when. Mr Brown needs now to focus as single-mindedly on Afghanistan as he did, for a while, on the global economic downturn.