A former French counterterrorism judge has raised a question Western governments should have asked years ago: What did Iran’s rulers gain when Europe and the United States weakened an organized force opposed to them?

Marc Trevidic’s account matters because he speaks from experience inside the judicial system. Discussing France’s 2003 proceedings against members of the People’s Mojahedin Organization of Iran, or MEK, he recalled a period when Paris pursued valuable oil and gas agreements with Tehran while the Iranian authorities sought action against their opponents.