David Grann has been a staff writer at The New Yorker since 2003. He is the author of “The Lost City of Z: A Tale of Deadly Obsession in the Amazon,” and his stories have appeared in several anthologies, including “What We Saw: The Events of September 11, 2001”; “The Best American Crime Writing,” of 2004, 2005, and 2009; and “The Best American Sports Writing,” of 2003 and 2006. A 2005 finalist for the Michael Kelly Award for the “fearless pursuit and expression of truth,” Grann has also written for the New York Times Magazine, The Atlantic, the Washington Post, the Boston Globe, and the Wall Street Journal.

For a moment, he was obscured by the Havana night. It was as if he were invisible, as he had been before coming to Cuba, in the midst of revolution. Then a burst of floodlights illuminated him: William Alexander Morgan, the great Yankee comandante. He was standing, with his back against a bullet-pocked wall, in an empty moat surrounding La Cabaña—an eighteenth-century stone fortress, on a cliff overlooking Havana Harbor, that had been converted into a prison. Flecks of blood were drying on the patch of ground where Morgan’s friend had been shot, moments earlier. Morgan, who was thirty-two, blinked into the lights. He faced a firing squad.

The gunmen gazed at the man they had been ordered to kill. Morgan was nearly six feet tall, and had the powerful arms and legs of someone who had survived in the wild. With a stark jaw, a pugnacious nose, and scruffy blond hair, he had the gallant look of an adventurer in a movie serial, of a throwback to an earlier age, and photographs of him had appeared in newspapers and magazines around the world. The most alluring images—taken when he was fighting in the mountains, with Fidel Castro and Che Guevara—showed Morgan, with an untamed beard, holding a Thompson submachine gun. Though he was now shaved and wearing prison garb, the executioners recognized him as the mysterious Americano who once had been hailed as a hero of the revolution.

It was March 11, 1961, two years after Morgan had helped to overthrow the dictator Fulgencio Batista, bringing Castro to power. The revolution had since fractured, its leaders devouring their own, like Saturn, but the sight of Morgan before a firing squad was a shock. In 1957, when Castro was still widely seen as fighting for democracy, Morgan had travelled from Florida to Cuba and headed into the jungle, joining a guerrilla force. In the words of one observer, Morgan was “like Holden Caulfield with a machine gun.” He was the only American in the rebel army and the sole foreigner, other than Guevara, an Argentine, to rise to the army’s highest rank, comandante.

After the revolution, Morgan’s role in Cuba aroused even greater fascination, as the island became enmeshed in the larger battle of the Cold War. An American who knew Morgan said that he had served as Castro’s “chief cloak-and-dagger man,” and Time called him Castro’s “crafty, U.S.-born double agent.”

Now Morgan was charged with conspiring to overthrow Castro. The Cuban government claimed that Morgan had actually been working for U.S. intelligence—that he was, in effect, a triple agent. Morgan denied the allegations, but even some of his friends wondered who he really was, and why he had come to Cuba.

Before Morgan was led outside La Cabaña, an inmate asked him if there was anything he could do for him. Morgan replied, “If you ever get out of here alive, which I doubt you will, try to tell people my story.” Morgan grasped that more than his life was at stake: the Cuban regime would distort his role in the revolution, if not excise it from the public record, and the U.S. government would stash documents about him in classified files, or “sanitize” them by concealing passages with black ink. He would be rubbed out—first from the present, then from the past.

The head of the firing squad shouted, “Attention!” The gunmen raised their Belgian rifles. Morgan feared for his wife, Olga—whom he had met in the mountains—and for their two young daughters. He had always managed to bend the forces of history, and he had made a last-minute plea to communicate with Castro. Morgan had believed that the man he once called his “faithful friend” would never kill him. But now the executioners were cocking their guns....