Helen Clark says joining the United States' so-called 'War on Terror' was the right thing to do at the time - but the effort was derailed by the invasion of Iraq, and ultimately hasn't been a success.
Clark was Prime Minister of New Zealand that morning in September 2001 when two planes crashed into the World Trade Centre buildings in New York, and a third into the Pentagon, where the US Department of Defence is headquartered.
As the chaos unfolded in the early hours of what was September 12 here in New Zealand, Clark was on a flight to Hong Kong, on her way to Rome. Internet news was still in its infancy at the time, and certainly wasn't available on the primitive cellphones of the day.
Televisions at the airport in Hong Kong showed pictures of smoke billowing out of the towers, but Clark was "operating in a fog with not a lot of information" and told The AM Show on Friday it wasn't clear to her exactly what was going on.
"I was sitting in the transit lounge in Hong Kong - this is before the smartphone and everyone sending Whatsapp messages - looking at this strange screen - no sound, no interpretation, smoke coming out of the towers," she explained, a day ahead of the 20th anniversary of the attacks which killed about 3000 people.
She got on her connecting flight to Rome, but "somehow, even 20 years ago, my head of department Mark Prebble was able to get through to me on the plane and tell me the bare bones of what had happened".
"We agreed to talk when I landed, and I very quickly made the decision to turn around and come home."
The blame was immediately placed on Osama bin Laden's al-Qaeda terror group, which was being sheltered by Islamist fundamentalist group the Taliban, which had recently gained control of Afghanistan.
The Taliban said if the US could provide proof bin Laden was behind the coordinated attacks, they'd hand him over. The US rejected the offer, then-President George W Bush saying there was "no need to discuss innocence or guilt - we know he's guilty".
When the US led an invasion of Afghanistan in October, Kiwi troops were with them. In the following two decades, 10 New Zealand soldiers based in the mountainous Asian nation lost their lives. The deployment ended earlier this year, ahead of the US pullout which led to the Taliban returning to power.
The Associated Press tally of the dead includes more than 47,000 civilians, about 7500 foriegnmilitary personnel and contractors, 66,000 Afghan military and police personell, 51,000 Taliban, hundreds of aid workers and dozens of journalists.
Clark said joining the international effort was the right call at the time, and she has no regrets.
"I don't think we could have done anything differently, and presented with that choice it's a no-brainer, right? Is it a medieval theocracy which harbours terrorists which are capable of mounting assaults anywhere in the world, or is it a country that you've got a long history of friendship with, not withstanding the row over nuclear weapons?
"I think what we did was appropriate. But as I say, you can replay the record and see the steps at which the international response could have been different in the following years, and perhaps with better results… I guess hindsight is a wonderful thing."
The biggest mistake, in her view, was the US-led invasion of Iraq less than 18 months later. New Zealand refused to join in, saying - unlike the Afghanistan mission - it wasn't sanctioned by the UN.
"Some in the George Bush Jr administration regarded [Iraq] as unfinished business from his father's time. That distracted attention away from Afghanistan, which needed long-term investment in its development to ever be secure."
Another misstep was the US rejecting overtures from the Taliban early in the war to sit around the table.
"And here we are 20 years later with the same medieval theocracy back in power in Kabul. The only difference seems to be they've worked out how to use social media."
Despite no attacks on the same scale as 9/11 on US soil in the years since, Clark said the Afghanistan mission can't be considered a success.
"For a number of years the Taliban have been in control for quite substantial parts of the country, but they didn't have the strength to take and hold districts and regional capitals - or Kabul itself - until the announcement of the pullout, which emboldened them.
"In that time also, foreign terrorist groups have taken up residence in Afghanistan again and seem to pretty much have a licence to operate now."