General Sir Michael Walker, chief of the defence staff, on Saturday also said, in an interview with the Sunday Times, that Britain and the US will have to make do with a less-than-perfect outcome from the US-led war.
The March 2003 invasion of Iraq to topple Saddam Hussein initially helped to attract new recruits and lift morale in the armed forces, despite being unpopular among the British public, said Walker.
"There was an understanding by members of the armed services that this was not an all-hands-up, popular event across the country," he said.
"But I think at that stage they were able to decouple in their own minds, as I was, the fact that the country was not necessarily behind the strategic decision to go to war, but once our boys and girls were out there, doing their various things, they would support them in that role," Walker said.
"Now I think that's shifted a bit, if I am absolutely honest. Some of the opprobrium attached to the war is also attached to the fact that the armed services are taking part in it. We are, if you like, guilty by association with a decision to go to war that not the whole of this country enjoined."
Conditions for withdrawal
Asked what conditions are needed to withdraw from Iraq and whether the conflict there is "winnable", the general said: "Winnable is the wrong word. I think what it is, is that there is a 'my glass is half full'."
"We are, if you like, guilty by association with a decision to go to war that not the whole of this country enjoined"
Sir Michael Walker,
UK chief of the defence staff
The Sunday Times said the "half full" comment indicated that Britain's ambitions for the violence-plagued country had been scaled down.
"I have no doubt that, provided we can keep the training and the security sector reform going, and providing some of the reconstruction will continue at the present rate, we'll reach a point where we can see an Iraq that is self-governing, providing its own security and has a democracy of the form that the Iraqis want," said Walker.
But he refused to be drawn on the question of how long British troops would be required to stay on the ground, reiterating the official line that it is up to the Iraqi government to decide.
"I would hope that as we go through the political process, the referendum and the next set of elections in December, it will become clear how secure the political machinery in Iraq is becoming," said Walker.
"If the elections are successful, and if a fully elected government gets in with a genuine mandate, then I think that confidence will overspill into all areas of progress.
"That will allow their own security forces to continue to develop from where they are at the moment, and at some stage in the future they will feel confident enough to take on their own management of the security in their country."