Greenberg gained visibility when he clashed with Mahmoud Ahmadinejad over the Iranian president's denial of the Holocaust. On September 20, 2006, the Council on Foreign Relations hosted a small meeting of select council members with Ahmadinejad, who began by saying that we need to "continue studying" whether it happened. According to David E. Sanger, Chief Washington Correspondent for The New York Times, Greenberg listened for fifteen minutes while Ahmadinejad continued talking about the Palestinians, World War II, and if the Holocaust killings had happened at all. Sanger writes,
