Audiences expecting to see a movie about World War II may have been surprised by how much "Oppenheimer" is instead about closed-door politics. One framing device Nolan uses to tell the story is the same one that authors Kai Bird and Martin J. Sherwin employ in the book: The hearing during which Oppenheimer's security clearance and loyalty to America were called into question. As in "American Prometheus," the film ultimately remains ambiguous about whether or not Oppenheimer was ever officially a member of the Communist Party, though both seem to put more stock in the idea that he was most likely a leftist FDR supporter who had associations with card-carrying communists.
In "American Prometheus," Bird and Sherwin quote Oppenheimer as claiming he was so apolitical and disinterested that he didn't know the stock market had crashed in 1929 until Ernest Lawrence (Josh Hartnett) told him six months later, and didn't vote until 1936. His support of left-wing causes — particularly of antifascists fighting Franco during the Spanish Civil War, Jewish scientists escaping the Nazis, and the support of union workers in California — began around 1934 and continued until roughly the time he became involved in war efforts. Oppenheimer insisted he never joined the CP, but admitted in writing during background checks that he'd been, "a member of just about every Communist Front organization on the West Coast."
Additionally, his brother, sister-in-law, girlfriend, wife, best friend, and several of his students and eventual employees were, at points in their lives, active communists. But whether radical ideas or associations with political parties are incompatible with patriotism is one of the larger questions Oppenheimer was forced to contend with.
