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.
I got intercepted by a call from also then a colonel, Telford Taylor, and, uh, would I come and talk to him? And he said, "Look, I'm going back to Nuremberg, I've been over there, the International Military Tribunal is already in process, Justice Jackson is there, we're winding up that trial but I'm going to take over after him, and we're going to set up a whole series of subsequent trials. And I'm going to be in charge, and I need staff, and I've heard about you and I'd like you to come with me." And I say, "What have you heard about me?" And he said, "I heard that you're occasionally insubordinate." And I said, "That's not correct. I'm usually insubordinate, because I will not follow orders that I know are stupid." I said, "But I've also been checking up on you, and I know you're a Harvard man, I know your background, and I don't think you're going to give me stupid orders, and if you don't you couldn't get a better man." He said, "You come with me." So I said, "Fine," I called up [Colonel "Mickey"] Marcus, I said, "Marcus, I'm off that army junket, I'm going out with Telford Taylor." And, uh, so I did, still intending to have nothing but a good time and enjoy myself and get even with the U.S. army. Well, it turned out that Telford was an excellent lawyer, uh, and he was much engrossed in the whole problem of who do you try after you've tried these few leading Germans, when you have the entire hierarchy of German life also responsible: the ministries, the industrialists, the generals, the SS people, uh, the doctors, the lawyers, uh, all of them conspired together to make it possible for Hitler to do what he was doing. And Telford's approach was, we have to reach out into all these segments of German society in order to demonstrate how it really worked. And it was quite fascinating and I really took the work quite seriously, and I said, "All right, what do you want me to do?" He said, "Well, where do I begin? Where do I go?" I said, "We got to get the evidence." I said, "I'll go to Berlin, that's where all the evidence is." So he said, "You go to Berlin, set up an office in Berlin." So I went to Berlin, my wife I had to leave behind. Uh, and, uh, there I set up the Office of the Chief of Counsel for War Crimes--Berlin Branch.