The West must accept that the war in Syria is lost, the former head of the British armed forces said as he claimed ministers rejected a plan he had drawn up to oust President Bashar Assad.
Former chief of the defence staff General Lord Richards said there was no political appetite for the year-long campaign which would have been required to defeat Assad when he proposed it in 2012.
The increased involvement of both Vladimir Putin's Russia and Iran meant that the West now had to concentrate on ending the conflict and making sure it had a strategy so it did not also "lose the peace".
"There is no clear strategy in the West, sadly. We are up against someone in Putin and Assad who have a very clear strategy and they are outsmarting us at the moment," he said.
The focus now must be on alleviating the humanitarian disaster and combating Islamic State, rather than supporting the rebels opposed to Assad.
He told the BBC Radio 4 Today programme: "I am afraid we have lost the war, but that does not mean, if we get the right strategy, that we need to lose the peace. I think that's what Western politicians and statesmen must now focus on."
The plan was put forward in 2012 and there was another opportunity for action in 2013, but MPs defeated then prime minister David Cameron's attempt to win support for air strikes against Assad's chemical weapons capabilities.
General Lord Richards said: "We offered a credible military strategy that would have taken about a year to execute. We were told we didn't have a year 'so that plan's not very good' and I remember saying at the time 'Well, if you don't want to do this' - which was to do it properly, to be frank, 'with clout and don't dribble' is a very useful military adage - I said 'Well, we had better start thinking about backing Assad because the result of not doing it properly could be the worst of all worlds'."
He said "there was a moment then when we could have dealt with him" but "we have to be pretty grown-up about this and say 'that moment has gone, the conditions on the ground, the political conditions in Syria, have now changed'".
The former military chief said the plan was "well thought-through, but at the time there wasn't any appetite, post-Iraq, for that sort of action".
Now the focus must be on ending the conflict, he said.
"We all are horrified about the humanitarian situation in Syria. If we do not bring this war to a close, that will go on for years yet. I think sometimes Western liberal opinion, and even dare I say me, have been very hypocritical.
"We wring our hands about the humanitarian situation but we have never been prepared to resolve it."