Ash Carter is a physicist and a defense-policy expert, having served in government periodically for decades. He was secretary of defense from 2015 to 2017. He has spent his academic career at Harvard, where he is today. In this “Q&A,” Jay Nordlinger asks him about some of the biggest issues: nuclear proliferation, North Korea, Iran, the size of the U.S. military. He also asks about the relation between our servicemen and the general American population. Is there too great a gulf between them? Do people sentimentalize our military? Is it okay to say “Thank you for your service”?