So-called “alternate history” has a long tradition, especially within literature. Speculating about what would have happened had various wars turned out differently is a particularly rich intellectual parlor game.

A notable example in this vein is Philip K. Dick’s 1962 novel “The Man in the High Castle”, which explores a parallel universe in which the Nazis won World War II and a conquered United States was divided by Germany and Japan. Adapted for the small screen by Frank Spotnitz, it recently enjoyed a four-season run on Amazon.

But what about wars or acts of massive destruction, nonexistent in fact, that were perhaps averted by small choices with much larger consequences? The latest effort from award-winning filmmaker Steve James, the documentary A Compassionate Spy, elicits deep-seated considerations of just such alternate realities.

Telling the story of a young American physicist who, in the shadow of World War II, passed along nuclear secrets to the Soviet Union, this fascinating work — part napkin-sketched Cold War history but, in large measure, a drama rooted in family secrets and regrets — will fire up the synapses of viewers whose passion for film intersects with history.

Born in New York City in 1925, Ted Hall was an overachiever from an early age, showing prodigious talent in science and mathematics. He transferred to Harvard University at 16 years old and graduated in only two years. Then, at only 19 years of age, he was recruited to work at Los Alamos on the top-secret Manhattan Project, a research and development program that would produce the first atomic bomb.

Hall was unnerved by the American bombings of Hiroshima and Nagasaki, in Japan. After the war — both motivated by strong personal feelings that an American monopoly on nuclear weapons was dangerous, and egged on by close friend Saville Sax — Hall gave detailed information regarding nuclear technology that expedited Soviet development of the atom bomb by at least five years, if not more.

Various FBI investigations would keep things tense but never lead to actual consequences (perhaps, the movie intimates, because Hall’s brother Ed was a later architect of the American military’s intercontinental ballistic missile program). With hindsight, though, and a bird’s-eye view of Joseph Stalin’s treatment of the Russian people, Hall would eventually arrive at a more conflicted place about his actions, if not outright regret them.

From his breakthrough film Hoop Dreams and 2011’s The Interrupters on through his last two long-form works for the small screen, America to Me and City So Real, many of James’s projects have a connection to his adopted home of Chicago. A Compassionate Spy is no different.

Hall left Los Alamos in the fall of 1946. He obtained his master’s and doctoral degrees at the University of Chicago, where he met his wife, Joan Krakover. Their relationship forms the spine of the film, recounted in interviews with her and the couple’s children. Hall would pass away in 1999, after complications from Parkinson’s and renal cancer.

James includes interviews with several figures who serve as narrative guides, including Joseph Albright and Marcia Kunstel, the co-authors of the 1997 book “Bombshell: The Secret Story of America’s Unknown Atomic Spy Conspiracy.” But the director seems a bit unsure of how to best integrate this material.

The true core of A Compassionate Spy remains its exploration of family histories and fraught dynamics (including that of the Saxs), rendered ever more complicated by hidden choices. It is here that the film most readily connects. James winds his way through the story of the fallout of Hall’s decision. In particular, how the death penalty cases against Julius and Ethel Rosenberg (accused of committing far less serious offenses than Hall) weighed heavily on he and Joan, even if their kids didn’t know it at the time. In exploring this real history, James invites viewers to ponder both an eye-opening alternate one, while also musing on how children handle the legacies of their parents.