Apr.03 Former U.S. Defense Secretary James Mattis says he can never rule out going back into public service. Mattis says it is time to have new leaders with fresh ideas to step up though. The retired general offers tips on how to be a great leader. Mattis appears on the latest episode of "The David Rubenstein Show: PeertoPeer Conversations." The interview was recorded on Dec. 5, 2019 in Dallas.