After receiving a gift of J. Robert Oppenheimer's speeches from Tenet star Robert Pattinson, Nolan decided to make a film about Oppenheimer and the making of the atomic bomb. Years prior, he read the nonfiction book American Prometheus, which several directors had unsuccessfully tried numerous times to turn into a film, and was fascinated by the enigmatic Oppenheimer. He wanted to adapt the book and tell Oppenheimer's story for years, but decided to make it after the transition of movie studios.
In an interview with The Hollywood Reporter, Nolan said “When you read the words of people [Oppenheimer and those in positions of power] speaking at that time, you see them wrestling with the implications and the consequences of what’s happened and what they’ve done." That was the basis on which Nolan wrote and directed the film. He wanted to explore the moral implications of creating and using such an innovative but destructive weapon. Having explored those themes in previous films, he wanted to tell the story of what truly happened (through an objective point of view) during the 1940s as a cautionary tale of the dangers of nuclear proliferation and the use of nuclear weapons. He feared that the bombings of Hiroshima and Nagasaki in Japan during World War II opened up a proverbial Pandora's box that has negatively impacted the world.
Likewise, Nolan aimed to take a complex view of Oppenheimer as a man. He thought that portrayals of him in the press and in other media had been two one-sided and lacked nuance. He wanted to paint the picture of a regretful man who devoted much of his life to a cause but had grown disillusioned with it over time. More than anything, though, the film was meant to be a warning about the ease with which nuclear weapons are discussed and used.
