Sebastian Junger and Nick Quested, as two of the world's most acclaimed war documentary filmmakers, have witnessed the horrors of mankind.

Their films include Restrepo , the 2011 Oscar-nominated documentary depicting the year Junger and the late British photojournalist Tim Hetherington were embedded with a US Army platoon in Afghanistan's Korengal Valley, considered one of the most dangerous postings in the US military.

American Junger and UK-born Quested, in what could be their last war film, have turned their attention to the terror group Islamic State (ISIS) and the Syrian civil war with Hell on Earth: The Fall of Syria and the Rise of ISIS .

"We both felt it was possible to make a 90-minute survey of the Syrian civil war to explain how it started, why it got so bad and why ISIS came out of this situation," Junger said.

The duo said they hoped there could be peace in five or 10 years' time in Syria but it would take Russian involvement.

"I think you would have to bring the Russians in on that," Junger said.

"You would have to crush ISIS. Bring the Russians in and figure out a partition like they did with Bosnia which requires Russian co-operation."

Filmmaker Sebastian Junger. (AAP)

The filmmakers said despite the bleak subject matter, there were times they were pleasantly surprised.

"Finding humanity in these dark places," Quested said.

"People with sunny dispositions and generous natures."

Part of the documentary focuses on a Syrian family living in hellish conditions and how they still manage to smile and remain positive.

"I wrote a book about this and it seems to me adverse conditions produce positive social behaviour in humans," Junger said.

"Situations that are safe and comfortable allow for people to act selfishly.

"In times of hurricanes, civil wars, the blitz in London, people recall those times with enormous fondness because it brought out the best human behaviours."

The filmmakers also interviewed Donald Trump's fired national security adviser General Michael Flynn, who has been central to an investigation into Russian interference into the US election.

"We interviewed him in March last year," Quested said.

"It was a long and involved interview and we wanted that voice in the film that said, 'You have to be aware of politicians using refugees for political gain'.

"We think it is even more poignant now that Michael Flynn subsequently changed his opinion. That's why we kept the interview."

Junger said the pair would likely move away from war and terrorism as subjects for their films.

"I don't want to be involved in stuff like that again," he said.

"I stopped war reporting a few years ago. I don't think I want to go into another film that is violent and graphic like this one was.

"I feel like I understand war to the extent that I want to understand it and I just became a father for the first time. My wife and I just had a little girl."

The film airs in Australia on National Geographic tomorrow at 7.30pm AEST.