Terrorism remains an ever-present threat but key weapons against it are timely intelligence and a free and tolerant society.
That's the view of former prime minister John Howard and ex-intelligence chief Duncan Lewis as the world marks 20 years since the September 11 attacks on the United States.
Nineteen al Qaeda militants hijacked four planes, flying two of them into the twin towers of the World Trade Centre in New York and one into the Pentagon just outside Washington.
A fourth plane crashed in a field in rural Pennsylvania.
Almost 3000 people died, including 10 Australians, and thousands more were injured.
Mr Howard doesn't believe al Qaeda has the capacity for such an attack these days, but says there is the prospect of a similar organisation "regenerating" now the US and partners have pulled out of Afghanistan.
"Terrorism has never gone, it is an ever-present threat," Mr Howard told a forum this week.
"The best weapon we have against terrorism is timely intelligence."
The former prime minister was in Washington on the day of the attacks.
He recalls the greatest fear on the day was that it would be a series of attacks across the world, including Sydney.
Having been ushered to a bunker beneath the Australian embassy, the next major decision was how to respond to the attacks.
"I certainly have no regrets about committing our forces to help the United States get rid of al-Qaeda - it was the right thing to do and it had overwhelming support in Australia."
Working hard at preserving freedoms within Australia and ensuring security alliances remain solid were key, he says.
Former ASIO boss, national security adviser and SAS commanding officer Duncan Lewis agrees good intelligence is central to tackling the terrorist threat.
The inquiry which followed the September 11 attacks pointed to a failure to fully understand an attack was imminent, he said.
But since then the world's intelligence-gathering machinery has greatly improved.
"While not publicly visible, this capability has done much to protect our communities from terrorist attack," he says.
Australia is one of the Five Eyes intelligence partners with the United States, UK, Canada and New Zealand.
"We must, however, continue to invest in our intelligence capability," says the Australian National University professor in national security.
"As a middle power, Australia must know what is going on around us to enhance our strategic decision-making and reduce the chance of surprise."
Like Mr Howard, he says building a more cohesive society is also key to reducing the prospects of attacks.
Prime Minister Scott Morrison said Australia would be standing with "our great friend and ally" the US on Saturday, but it was also a deeply personal tragedy.
"First and foremost I just think about the terrible tragedy that took place in so many people's lives," he told 2GB.
