Six months after proclaiming a new commitment to the war in Afghanistan, President Barack Obama is under growing pressure to make what would amount to a U-turn in US policy and scale back America's commitment to a conflict that many experts – and a majority of the public – now fear may be unwinnable.

The debate, which divides Mr Obama's most senior advisers, was thrown into stark relief by the leaked report of General Stanley McChrystal, the commander of US and allied forces in Afghanistan, warning that the war might be lost within a year without a further boost in troop strength and a major change in strategy to combat the spreading Taliban insurgency.

General McChrystal's bleak assessment coupled with Washington's frustration with the Afghan leader Hamid Karzai and the fraud-ridden election over which he presided, has reignited a rift between Vice-President Joseph Biden and Hillary Clinton, the Secretary of State, over how the war should be waged. It has also left Mr Obama facing a fateful choice: whether to go along with his generals and send yet more troops, or stand current policy on its head.