The 11-year campaign has so far cost 438 British lives. Lord Ashdown writes in The Times today urging Britain to pull troops out of the country as quickly as “decently” possible.
He says the campaign is “not worth the life of one more soldier”.
“It is now crystal clear that we have lost in Afghanistan,” said Lord Ashdown.
“We have succeeded in only one thing; albeit the big thing we first said we went to war for – driving out al-Qaeda. In almost all other tasks we set ourselves, especially the establishment of a sustainable state, we have failed.”
The cost of life has stretched further than the battlefield. An inquest has heard how Rifleman Allan Arnold from Cirencester was found hanging in 2011 while he was home on leave.
An inquest into his death heard how he struggled to come to terms with the deaths of five comrades killed in a roadside blast in Sangin, two years earlier.
The 20-year-old soldier with 2 Rifles left a note saying he could not cope any more and apologised to his mother.
Another Gloucestershire life lost to the conflict has had his name added to a Nailsworth war memorial.
Mark Chandler, shot dead in Afghanistan, has been added to the Church Street monument.
At the time of his death Captain John Mercer, fire support commander on the patrol, described the former Archway School pupil as “the perfect soldier”.
Lance Bombardier Mark, 33, of the 3rd Regiment Royal Horse Artillery, was killed in Nad-e Ali, Helmand, during a patrol that ended under intense fire back in June 2010.
Captain James Townley, who grew up near Glastonbury, was on his third tour of duty when he was killed on Friday, September 21 – the day before his 30th birthday.
He had been serving with 21 Regiment Royal Engineers when he died at Camp Bastion in Helmand Province.
He is the only former Bruton Sexey’s School pupil to be added to its memorial since the Falklands conflict in 1982.
Lord Ashdown’s calls come as Government plans are finalised to exit Afghanistan by 2014.
As a former member of the Special Boat Service, he argues the word “defeat” is inappropriate because of the individual success in battles fought by British and Nato troops against the Taliban.
This year, 61 coalition soldiers have been killed in green on blue attacks – the Afghan National Army or police. Of those, 14 have been British.