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.
As the camps were about to be liberated, the Germans tried to move the inmates out, those who were still able to walk or to work. They left those behind to be killed or to die who were too sick. But they marched them out. And they were marching, I think it was from Flossenbürg to march to Dachau, or one of the camps. And, they took them through the woods and they marched at night, and if anybody faltered on the way, they were immediately shot; if anybody paused to try to pick up a potato or to eat a root or something, they were shot. And I was able to follow this trail through the woods, uh, of mass grave--10, 20, 30, 50 killed, you know. And I would get the nearest farmer to say, "Dig them up." They would say, "Oh yes, we heard firing there last night, there was shooting going on." "Where was it?" "Over there in the woods." And, uh, I would say, "Let's go." And we'd go out to the woods and there would be a newly dug-up place, and I'd say, "Get some shovels," and I'd stop some Germans on the street, "Take those shovels, dig 'em up," and we'd dig up the bodies of, uh, people who'd been obviously shot through the head. Usually the top of the skull is blown off, uh, shot probably kneeling from the back, uh, some of them were tied still, uh, you know, just lightly covered over with, uh, you know, six inches of dirt, something like that. So I could follow the trail of crime being committed all along the way.