For three-quarters of a century, Auschwitz has been a byword for the evil that humans can do, for the terrors wrought by tyrants against people whose only crime is to have been of a certain ethnicity or religious tradition and in the wrong place at the wrong time. During the five years that, night and day, it served as a factory of death, it was not a place that people entered willingly. Except, that is, in the case of one man, a Polish officer and resistance fighter named Witold Pilecki, who stepped forward and volunteered to be imprisoned in that hell.

Jack Fairweather first heard of the story of Pilecki (pronounced Piletski) in the company of a fellow war correspondent at a Long Island dinner party eight years ago. So recounts the British journalist in his new book, The Volunteer: One Man, an Underground Army, and the Secret Mission to Destroy Auschwitz, writing, “The idea of a few souls standing up to the Nazis comforted us both that night.” Having told untold stories himself on the front lines in the Middle East, he adds, “I was equally struck by how little was known about Witold’s mission to warn the West of the Nazis’ crimes and create an underground army to destroy the camp.”

Auschwitz was really three camps in one. It was a prison camp for some military and political prisoners, who stood at least some chance of surviving it. It was a slave-labor camp for some Jews, captured Russians, and other unfortunates who could be put to work, with conditions ranging from the brutal to, as Steven Spielberg’s film Schindler’s List depicts, the relatively (and always tentatively) protected. And, most notoriously, it was an extermination camp. It was the last guise that set the tone for the entire complex; as Fairweather writes, when Pilecki arrived, having convinced his fellow resistance fighters to allow him to enter the camp in September 1940, a full year after the Nazi invasion of Poland, the German commandant barked, “Your Poland is dead forever, and now you are going to pay for your crimes through work.” Pointing to the giant smokestack of the death camp’s crematorium, he added, “The chimney is your only way to freedom.”

It was not. Pilecki kept careful records of all that he saw, hiding them, dispatching them to the outside world through an astonishingly successful underground railway. The dangers were ever present that guards would discover him, that an inmate seeking favor might inform on him, that illness and overwork would fell him. Yet just a month after Pilecki entered Auschwitz, his reports were making their way to Warsaw, and from there to London—a fact that extends the story of what the Allies knew when and that complicates their decision not to bomb the railway lines into Auschwitz or otherwise interfere in the Nazi death machine.

Fairweather entered Witold’s world with the knowledge that he had found a little-known story in a war that has filled whole libraries. The task before him, he discovered, was immense, and it took as many twists and turns as a hand-dug tunnel, involving going through thousands of prisoner testimonies, constructing and reconstructing hundreds of grim episodes. “At times, I could almost feel as if I were there, moment by moment, seeing events through many eyes,” he says from his New York home. “More, I came to see the camp through Witold’s eyes, which was important in helping me understand his role as witness. In time, I thought it was an honor to see how those stories and events were interwoven, and I came to see my job as a kind of literary forensics.”

The stories thus interwoven, forming a vast testimonial, weren’t just taken from the prisoners’ points of view. Pilecki was careful to recount the role of the hated kapos, prisoners who, to curry favor with their captors, were sometimes as brutal as the SS guards. (One satisfying, if grisly, moment in Fairweather’s long narrative is the demise of a particularly hateful collaborator.) He tried to understand the motivations of the Nazis as well. Pilecki, Fairweather writes, early on came to the conclusion that “not everyone could be saved, either physically or spiritually.” Some prisoners, Pilecki determined, did all they could to ingratiate themselves with the kapos and save themselves. Others died of typhus or other epidemic diseases or starved; others simply abandoned hope. But many resisted in whatever ways they could: a medical assistant who falsified hospital records so that long-term patients wouldn’t be taken off and killed, a kapo who warned his charges that they were about to be gassed, earning him his own execution.

Fairweather’s groundbreaking book offers a critical account of events that are still controversial. For one thing, there is that question of just how early on in the war Winston Churchill and other Allied leaders knew of the existence of the death camps and the Final Solution they were meant to effect. “The tracing of Witold’s reports from the camp to Warsaw and London has not been done before,” says Fairweather. “Indeed, the fact that Pilecki was calling for the camp to be bombed as early as October 1940 raises one of history’s great what-might-have-beens. It sets back the clock on what the Allies knew.”

In a Poland that is deeply sensitive to the behavior of its citizens in World War II—a recent law makes it illegal merely to suggest that Poland may have played even a tiny part in the Holocaust—Pilecki’s story is itself divisive. Although some Poles, for instance, cheered as the Jews among them were driven from their homes and sent to ghettos or camps, most, as Fairweather writes, refused to participate or aid the Germans. Still, most also remained silent. “There are some in Poland today,” Fairweather says, “who would like to use Witold Pilecki as an example of their country’s unblemished resistance and heroism. But in fact, even in the resistance there were many anti-Semites, and Pilecki had to push back hard against them.”

Pilecki himself despaired, and he engineered a daring escape even as one of his resistance colleagues was making his way to London with still more extensive reports of the Nazis’ genocidal program. When that man arrived in London, as Fairweather notes, he discovered that the Allies “had no interest…in supporting rescue measures that might divert resources away from the war effort.” Pilecki himself was recaptured after the Warsaw Ghetto Uprising and spent time as a prisoner in Germany before the Allies liberated him. Returning to Poland, he soon fell afoul of a new terror in the form of a Stalinist satellite government that condemned him as a traitor.

His story might have died with him, but Fairweather’s years of research, interviewing, reading, and contemplation have rescued Pilecki, whom he calls an otherwise “average man,” from oblivion. The Volunteer is a central document in helping readers understand the everyday workings of the Nazi war against Europe’s Jews. And, as Fairweather quietly observes, it “obliges us to confront how we respond to evil in our own time.”

Gregory McNamee is a contributing editor.