One of the ADF's most senior officers and the chairman of veterans' support group Soldier On, Lieutenant-General Leahy said it was important that all the soldiers, sailors and aviators who were involved in the conflict knew they did their job well and professionally, and urged them to reach out, get help and talk to people should they be feeling a sense of loss about the Taliban's retaking of the country.

"There will be a lot of servicemen and women out there now wondering what this [the Taliban takeover] means to them," he said.

"It was the leadership which put them in this situation and they [the defence personnel] did all we asked of them; they should be proud of what they've done, and their professionalism."

The rapid capitulation of the Afghan National Army over recent weeks was a major disappointment to Mr Leahy, given the enormous amount of money, resources and training which was directed there.

The Australian Federal Police, too, made a significant commitment to training local police services between 2009 and 2013, and those officers would be sharing the same sense of anger and frustration at their wasted effort.

However any Australian military presence within Afghanistan was untenable once the US withdrew, and Mr Leahy said he now feared for what the country may look like in a year's time, or even less.

"To me, the telling part here is we put in a lot of time, effort and training into the Afghan National Army and the security forces, and they've disappeared form the field of battle," he said.

"So I don't know what more we could have done."

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Lieutenant-General Leahy said Australia was correct in going into Afghanistan in October 2001.

"We had a clear mission, to get al-Qaeda and deny them a base of operations," he said.

"We came back to Afghanistan in about 2005 when Iraq became known as the 'bad war'. And in 2005, we didn't have a clear mission.

"There were ad hoc approaches at nation-building, and my major complaint is we didn't have a clear strategy of what we wanted to achieve.

"There was no thought as to what victory would look like; there was no real thought about what was the nature of peace in Afghanistan. And then we seemed to get in a set-and-forget phase."

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