Now he’s 95, and tired. “I’m getting old,” he said. “I’m running out of steam. I need help from young people.”

To be clear, nothing about Ferencz’s demeanor indicates a deficiency of steam. I met him outside of a convention center in Washington, D.C. on a sunny spring day, and as we went through a metal detector inside, he happily showed off his suspenders for the security guard to check. “How old are you? You get around pretty well,” the guard said. “For an old guy,” Ferencz replied. He pointed a thumb at me. “This is my girlfriend,” he added.

We sat on a bench in the sun, and there, he told me about the bodies at Buchenwald. “I saw crematoria still going, the bodies starved, lying dying, on the ground. I’ve seen the horrors of war more than can be adequately described.” He spoke clearly and without much emotion. I heard familiar phrases that stuck out from previous interviews I had read in preparation for our conversation. This was how he had learned to tell his story: straightforward, detached, honest but without too much detail. This, I think, is how he has survived 70 years of recalling exactly what it looks like when thousands of murdered Jews are laid out side-by-side, stacked in piles.

After fighting with an anti-aircraft artillery battalion in the U.S. army during World War II, Ferencz was assigned to General George Patton’s office and tasked with helping to establish a war-crimes division. This was not a typical mission, for one good reason: The army had never had a war-crimes division before.

As part of this effort, Ferencz joined the forces that liberated a number of concentration camps in what was then Germany, including Buchenwald and Mauthausen. He collected documentation: the number of bodies, and where they were located; the sanitary conditions of the camps; the files left behind by army officials, including ledgers recording who had died, and when. It was this evidence that eventually led to the speedy conviction of the Einsatzgruppen commanders. “I was able to rest my case after two days without calling a single witness—the top-secret documents were indisputable,” Ferencz said.

But without his intervention, these men may have never been taken to trial. “The case had not been planned,” he said. “When we discovered this evidence, I brought it to General [Telford] Taylor, and I said we have to put on a new trial, and he said we can’t.” The Pentagon had already planned its schedule of trials, Taylor said, and the war-crimes division faced staffing shortages and budget limitations.

“I said, ‘We can’t let these mass murderers go free—I have the evidence here in my hands’. And he said, ‘Can you do it in addition to your other work? OK, you be the prosecutor.’”

Looking back, this anecdote seems outrageous, suggesting that the trials following the most extensive genocide in human history were haphazardly assigned to young, newly minted prosecutors. But this is what’s so remarkable about Ferencz’s career: Again and again, he has been asked to establish law and order in situations that had never been dealt with before on such a large scale. If it sounds like the army was making up trial procedures as it went along, that’s because, well, it was.