Schulman, who is now a fellow at the bipartisan Center for a New American Security, described Mattis’s approach as “incredibly naive” and a reflection of a “lack of political astuteness” that she said was apparent during his tenure as Pentagon chief.

Others argued that Mattis’s silence amounts to a tacit endorsement of the president and his policies.

“Mattis saw his duty as preventing the worst from happening” when he was defense secretary, said retired Army Lt. Col. Jason Dempsey, who has written extensively about civil-military relations. “But he also legitimized the worst. He lent his honor and integrity to the Trump administration. He didn’t just give the president Jim Mattis’s credibility. He gave Donald Trump the military’s credibility.”…

“I don’t view it as unethical to write a book on general leadership principles, but he is setting himself up for a painful series of interviews . . . and a painful series of articles,” Feaver said of Mattis.