General James Mattis (Ret.), who served as secretary of defense for President Donald Trump, talks about why he left the post, how to become a great leader, if he would ever go back to working at the White House, and why he doesn't like the nick name "Mad Dog." Mattis appears on the latest episode of "The David Rubenstein Show: Peer-to-Peer Conversations." The interview was recorded on Dec. 5, 2019 in Dallas.