Ben was born in a small village in the Carpathian Mountains of Transylvania in Romania. When he was an infant, his family moved to the United States. Ben attended Harvard University, where he studied criminal law. Ben graduated from Harvard University Law School in 1943. He joined a US anti-aircraft artillery battalion that was training in preparation for an Allied invasion of western Europe. At the end of World War II in Europe, Ben was transferred to the war crimes investigation branch of the US Army. He was charged with gathering evidence against and apprehending alleged Nazi war criminals. He ultimately became chief US prosecutor in The Einsatzgruppen Case of the Subsequent Nuremberg Proceedings.
The office was the Third Army, Judge Advocate headquarters, which kept moving as the front kept moving up. It was either in Erlangen or it was in Munich or it was, uh, someplace else, and, you know, we had, usually took over a German Kaserne [barracks] and we had a room and a desk and a typewriter. So I would get back there with whatever notes I had, whatever documents I had, and write up a report. And, um, the reports would say: "On certain date U.S. army troops entered the camps of X"--let's assume Mauthausen, for example. "There the troops encountered the following scene: there were originally 50,000 inmates in the camp, there were 12,000 still alive, 10,000 had been marched out the day before. The camp officers were so-and-so. The crematoria were still going, there were so many bodies stacked in front of the crematoria. I took witness statements from 10 witnesses, they're attached as exhibits one to ten. Um, the suspected persons responsible for these crimes are so-and-so and so-and-so. Issue orders immediately to have them put on the CROWCASS list, the Central Registry of War Criminals and Security Suspects. Have them distributed to all members of the U.S. army, wherever any POWs are, compare them with this list, have them arrested, and report back to headquarters." So, the goal of my investigation was to describe what had happened, to collect credible evidence admissible in a court of law, which could be used to convict the persons responsible of a known crime under international law. That was the objective, and that's what we did.