Former Liberal Democrat leader Sir Menzies Campbell has warned of a "vacuum" over Afghanistan because of the length of time being taken by the US to make a decision on extra troops.
Sir Menzies said he would be "extremely surprised" if the strategy that emerged, while not setting a date for withdrawal, was not based on creating the right conditions for withdrawal.
He said precipitate action would cause regional instability, damage Nato and "very likely" return the people of Afghanistan to control by the Taliban.
Defence Secretary Bob Ainsworth, wrapping up the debate, pleaded with the British people to maintain their support for the mission in Afghanistan despite the bloody price paid by the Armed Forces.
He said the Taliban's strategy centred on sapping the will of the public but they would only succeed "if we lose our resolve".
Mr Ainsworth acknowledged there were sections of the population which questioned the continued commitment to the battle against the Taliban but insisted failure would leave Britain less safe.
He told the Commons: "I understand the strength of public feeling when confronted with the reality of fighting.
"But we can't afford to be half-hearted in Afghanistan and neither can we here at home."