Mr Howard was in the US on September 11, 2001, and was set to address Congress when Al Qaeda toppled the World Trade Centre towers.
See Chris Uhlmann's full report tonight on 9News at 6pm
Days later, flying home on Air Force Two, he said he spoke to then-Foreign Minister Alexander Downer.
"I think it was Alexander Downer who said in that conversation, why don't we invoke the ANZUS treaty," Mr Howard told Nine Political Editor Chris Uhlmann in an exclusive interview.
"We agreed then that was the right thing to do."
'When people say it is a failure they have to factor into that judgement, if they make a judgement, what might have happened if we'd done nothing'
Mr Howard said the evidence that the attacks had been carried out by Al Qaeda out of Afghanistan was "overwhelming" from the early stages.
And as the 20-year US-led occupation of the country ends in a lightning takeover by the Taliban, he denied the mission had been a failure.
"Judged by the original reason for going in, it was a total success because there was no like attack coming out of Afghanistan organised by Al-Qaeda," he said.
"When people say it is a failure they have to factor into that judgement, if they make a judgement, what might have happened if we'd done nothing."
He said the great bulk of soldiers who served in Afghanistan that he had spoken to had been glad for the opportunity, though he acknowledged that the families of the 41 soldiers killed in the country would have "a mixture of emotions".
"But can I say to them, they fought well, they fought bravely, they fought professionally, and the nation should always be in their debt," Mr Howard said.
"It was a proper and noble cause."
However, he was openly critical of the manner of the Western retreat from Afghanistan.
"It's hard to be other than critical of it," he said.
"I find it very difficult to accept in order to pull out and end military activity you have to pull out every last soldier.
"It should have been possible for the US, in consultation with allies, to maintain a smaller presence there for an indefinite period of time."
Watch Chris Uhlmann's full interview with Mr Howard in the video player above.