"The bomb, Dimitri. The hydrogen bomb," an exasperated US president reminds his Soviet counterpart in Stanley Kubrick's 1964 classic "Dr Strangelove or: How I Learned to Stop Worrying and Love the Bomb."

As if the bomb could ever be forgotten. At the height of the Cold War, when mutually assured destruction was at its crazed height, nuclear warfare found itself abstracted through humor -- perhaps the only reasonable way to treat something so fearsome. The bomb was a sick joke to be ridden, sometimes metaphorically, sometimes literally, a totem of mankind's ingenuity and stupidity riveted shut and let fly. To look at it head-on was to be blinded by its glare; the power it wrought was too absurd — and too close — to countenance. Best to send in the clowns.

It would have seemed inconceivable then, but the atomic bomb did fade out of mind. In the 21st Century, other existential threats have reared their heads. Yet the nuclear threat feels closer today than it has for generations. And it's precisely at this moment that Christopher Nolan is asking audiences to look it soberly in the eye.

"Our relationship with nuclear weapons is very complicated," Nolan told CNN. "The fear ebbs and flows. It's almost like humanity can only deal with one apocalypse at a time, and there's so many issues to worry about."

But worry many will after watching "Oppenheimer," his latest and perhaps greatest film to date. As the name suggests, Nolan has taken on the life of J. Robert Oppenheimer, the scientific genius and conflicted godfather of the atomic bomb. Working in IMAX, the director conjures an overwhelming, altogether haunting retelling of the story of the bomb's creation and its fallout across three taut hours that pushes the limits of the medium itself.

Nolan's screenplay -- written, unusually, in the first person -- drew from definitive biography "American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer" by Kai Bird and Martin Sherwin, but stops short of its forensic take on the physicist. Flitting between color and monochrome -- the director has described the former as a subjective and the latter an objective lens on events -- we mostly experience the narrative through Oppenheimer's eyes.

"We have to get the facts right. We have to be guided by history. But we're trying to give the audience an experience. We're trying to really entertain and engage in a meaningful way," Nolan explained.

"It's a subjective telling," he added. "We're trying to not judge the man. We're trying to experience things with him and understand."

Oppenheimer's piercing blue stare is inhabited by Cillian Murphy, the director's long-time collaborator and first-time leading man. "Oh, there was a man," Murphy said of his character. "I was so exhilarated to be given the opportunity -- it's kind of a dream part. But it's so multifaced and massive, so you just dive in."

Murphy plays Oppenheimer from a young student in Cambridge in the 1920s through to the post-war McCarthy era and beyond, without the aid of distracting de-aging CGI (the physicist died in 1967). Around him swirls a who's who of impeccably suited scientists, military men and politicians, along with the women in his life, wife Kitty (Emily Blunt) and lover Jean Tatlock (Florence Pugh) -- a vast supporting cast in the truest sense.

"Watching Cillian and Chris together, it was two people at the height of their abilities," said Matt Damon, who stars as Leslie Groves, the brusque general overseeing the Manhattan Project.

"It felt the most like watching when Spielberg did 'Saving Private Ryan,'" he added, "watching Chris Nolan with a story that's worthy of his vast ability. Everybody could feel it, so everybody came with everything they had and was like, 'Alright, how do we support this?'"

A threat renewed

For most of Nolan's considerable cast, Oppenheimer's invention set the backdrop for their upbringings. "This was our existential threat growing up," recalled Robert Downey Jr., who plays Lewis Strauss, founding commissioner of the U.S. Atomic Energy Commission and later a political operator.

"We did grow up at this time, but it's almost too much to engage with, so you push it out," said Damon. "It's like we've all been living under this sword of Damocles for 80 years ... Our kids, it's not part of their experience to the extent that it should be."

Nolan knows this only too well. "I told one of my teenage sons what I was working on," he recalled. "He said to me, 'Well, does anybody really worry about that anymore?' This was a couple of years ago. By the time we finished shooting the film, unfortunately he wasn't asking that question anymore."

Russia invaded Ukraine days before cameras started rolling, noted Murphy, and it was under that cloud that filming began in New Mexico, at a stand in for the Los Alamos HQ for the Manhattan Project.

The film has taken on troubling relevance, especially the passages set post-war, documenting an arms race that Oppenheimer warned against. Its examination of his legacy, from the physicist's perspective and the people around him, forms a devastating and complex coda.

Today, the ethical debate surrounding the bomb remains undimmed. But even if a war involving a nuclear power hadn't erupted in Europe in 2022, Nolan believes "Oppenheimer" contains crossover anxieties.

"(Nuclear war) is unique in the threats that we face, but it also relates to a lot of our relationship -- and particularly young people's relationship -- with technology and technological innovation," he said. "Because ultimately, the atomic bomb, awful though it is and was, it was an incredible technological innovation."

Eighty years after the Trinity Test, technological innovation looks like artificial intelligence (AI), he offered, and the potential dangers of "unleashing those things on the world."

"A lot of the scientists I speak to, a lot of the people who work in the tech sector I speak to, they worry about this," he added. "They see what they're doing as their Oppenheimer moment, and they look to his story to give some kind of guidance as to how the relationship between science, technology, government and society should function.

"I don't know whether Oppenheimer's story has answers to that," he added. "It certainly raises the questions, and in a way that I think we will find quite chilling."

Kubrick had us laugh at the bomb because it felt like an appropriate coping mechanism. Now Nolan's fierce and sincere contemplation will awaken a new generation to its horrors -- and that feels appropriate too.

For a director obsessed with the workings of time, the arrival of his latest movie is uncanny. But one senses the film will outlive this moment, and only burnish his reputation as one of the few blockbuster directors with the clout to pull off working at this scale.

"There's always been this mystique around Nolan," Downey Jr. shared. "The spring-into-summer that the first 'Iron Man' came out (in 2008), people were like, 'Oh, this will do until "The Dark Knight" comes out.' So, you're like, 'Oh, it's not a Chris Nolan.'"

"He's a master filmmaker, there's no denying it," he added, "and you don't get many of those per generation."

"I think this film is kind of the apotheosis of his talent as a director," said Murphy. "To have had the privilege of working with him over the years and seeing him move towards this, it's phenomenal to witness."

"Oppenheimer" is released in cinemas in the US and UK on July 21.