"A Compassionate Spy", by celebrated US filmmaker Steve James, hopes to reignite debate about nuclear weapons at a time of rising geopolitical tensions.

"Climate change and other issues have taken our attention away from that threat but it's always been there and it's coming back," James told AFP in Venice.

Ted Hall was just 19 when he was recruited to work on the top-secret Manhattan Project during World War II that led to the creation of the world's first nuclear weapon.

Sympathetic to the Communist cause, and fearing a future in which only the US had the bomb, Hall decided to pass designs to Moscow.

The story has been largely forgotten, even though Hall came clean in the last years of his life in the 1990s.

"Many people will no doubt conclude that he should not have done it, that his fears of the US becoming fascist or the US pre-emptively striking the Soviet Union were not grounded," said James, who is known especially for his landmark 1994 documentary "Hoop Dreams".

"But there's no question he did it for the right reasons -- he didn't do it for profit or fame, he did it because he had a genuine fear of what the US is capable of.

"And ultimately, we're the only ones who have dropped a nuclear bomb so it's not an unreasonable fear."

Although the FBI long suspected Hall of espionage, they were never able to find conclusive evidence.

But the tension for him and his family was almost unbearable, especially when two other spies, Julius and Ethel Rosenberg, were executed in the US in 1953.

The film makes clear the vastly different attitudes towards Russians in 1944, when the Soviet Union was a wartime ally, seen as heroically standing up to Nazism.

Hall later said he would not have done it had he known about the crimes of Joseph Stalin at the time.

"Maybe he was wilfully naive," said James. "But we have to remember, he was so young."