In the early hours of Aug, 21, Scott Cairns was struggling to fall asleep in his Damascus hotel room when, staring out the window, he saw what appeared to be fresh fighting to the east.
"At that point I didn't know what it was," he said in an interview with CBC News. "It happens on a regular basis …there is incoming and outgoing fire from Damascus, so there are constant thumps and explosions. It was only later, when I heard the reports, that I could put it together."
Cairns, a chemical weapons inspector in Damascus for the UN, later concluded that what he had witnessed from the safety of his hotel room was what is now believed to be the worst chemical weapons attack against civilians in a quarter century.
The timing, Cairns agrees, was odd.
The attack happened while his UN inspections team was on the ground to look into three prior allegations of chemical weapons use in the course of that country's more than two-year-long civil war.
In every instance, both the Syrian regime and the rebels blamed each other.
Then came that latest, most grotesque example, in the Damascus suburb of Ghouta. It, too, invited the same finger-pointing especially because of its timing.
"It's probably not in anyone's interest to launch an attack while inspectors are there," Cairns said. "Our job ultimately became to switch directions and investigate that location."
From there, it was a fast moving chain of international and often dramatic events involving the world's top leaders, that ultimately took Cairns back to Syria this week on another mission: to start the process of dismantling and destroying all of that country's chemical weapons capability.
Inspecting the Ghouta area so soon after an alleged attack was an unexpected challenge (and an international first), for the UN team. But in its scope it pales next to the task of ridding Syria of its vast stockpile of chemical weapons.
It's estimated Syria has some 1,000 tonnes of chemical agents and precursors, including VX, sarin, and mustard gas.
And though it denies responsibility for the Ghouta attack, the Syrian regime has nonetheless agreed to a UN Security Council resolution, passed last week, stipulating that its entire arsenal must be destroyed by the middle of next year.
The Organization for the Prohibition of Chemical Weapons, the small group that employs Cairns and is headquartered at The Hague, Netherlands, will lead the way.
An advance team, which includes Cairns, enters Syria today to begin inspecting the arsenal that the Bashar al-Assad regime has thus far declared.
Cairns will be in charge of the field teams visiting the disclosed sites and which will ultimately start destroying the material. Two other Canadians are involved in this phase, with more possibly joining later.
Ridding Syria of all its chemical weapons is "very difficult," Cairns said on Friday. "To do that in a permissive environment, or a country that's at peace, is extremely ambitious. To do that in a matter of months, or in a year, in a country that's in a full-blown war, borders on the unrealistic.
"The job we have ahead of us is Herculean in some ways, but I'm very much looking forward to making the effort, and working with my colleagues to make it happen."
The OPCW is hoping donor countries will help pay for what is expected to be a costly endeavour.
Canada has yet to earmark money specifically for the destruction of the weapons, but the foreign minister's director of communication points to a $2 million contribution it made earlier this year towards the initial inspections.
Officials say stopping production by Nov. 1 is a priority and might be done simply by taking sledgehammers to machinery, or filling empty shells with concrete.
Watch out for snipers
Cairns, a chemical weapons and munitions expert, is from Burlington, Ont., and grew up in a military family, often moving around the country.
He studied chemistry at the University of Manitoba, before landing a job at the Department of National Defence in Suffield, Alta. Between 2002 and 2007, he provided chemical expertise to military and government agencies, as well as training to armed forces in regards to chemical agents.
In 2008, he moved to The Hague to join the OPCW, and he is now among a handful with experience on the ground in Syria who will help guide this latest mission.
The key during his last visit as team leader, he said, was maintaining impartiality despite the competing interests — as well as all the compelling video of the attack that so shocked the world.
Just as challenging, then as now, will be remaining focused on the task at hand despite the obvious threats to the team's security.
In August, a sniper attacked their convoy within minutes of heading towards Ghouta. The incident occurred on the regime side of a no-man's land between Damascus and the suburbs.
"The first emotion that went through my mind was anger … I didn't like people shooting at my team."
He says he felt it "was more of a message sent to us than an actual assault … to try to get the UN to turn tail and leave."
But they didn't leave. They returned to Damascus to switch vehicles, and later that day they were in Ghouta collecting evidence.
"The physical evidence doesn't lie. It is even more impartial than we are," Cairns says.
The inspectors' report — and the information on the munitions used — was proof for many nations that the Syrian regime was behind the attack.
The report itself does not apportion blame. It only confirmed that sarin was indeed used in Ghouta.
But shortly after it was released, the inspectors were accused of bias by Russia, the Syrian regime's main backer, which apparently believes the rebels were behind the attack.
"I was there. I took some of those samples. I assessed the situation. I have experience in this, the mechanism under which we took the samples transport them and have them analyzed is extremely rigid with this impartial chain…[I'm] a hundred per cent sure," of the results, says Cairns.
"Everybody is entitled to their own opinions."
Canadians, a natural choice
As impartial as they tried to be, the inspectors were faced with real-life and often overwhelming stories of the events of that terrible attack.
"There [were] examples where small children had lost their mother and father, and all their siblings. So you look into their eyes and you see that they don't really know what they're going to do next.
"And it's a struggle, as I'm sure anybody that goes into those areas has, to maintain your professional limits because you want to do everything for everybody. But you just cannot.
"You focus on what you have to do … If we didn't do our one single task correctly in the very limited time we had, we would have failed those people anyway."
With his expertise, Cairns would surely have been part of that first mission. But he ended up at centre stage back then when it was agreed that none of the five permanent members of the Security Council would send inspectors for that initial round.
Canadians are a natural choice in such circumstances, says Cairns.
"We've had a reputation in the world for a long time as being the people to go to for peacekeepers, and we tend not to have any sort of overt agendas when we do these things.
"So I think the comfort level for a number of people was very high that Canadians were involved."
As he sees it, "I think we are truly helping people. These are horrible weapons. There's no reason to use them in this day and age."
"The fact that people were thanking us there was some kernel of yes, we were doing something good, we were helping these people.
"It's a balm to my conscience sometimes in the evening, and I know it's helping my team."