During former Defense Secretary James Mattis's MSNBC interview with Andrea Mitchell, he refused to criticize President Trump, even though she pressed him firmly to do so on various issues.

Mitchell tried to get Mattis to say Thursday if he thinks he is "normalizing" decisions Trump makes by not saying how he truly feels about them, such as when former Secretary of State Rex Tillerson was fired via Twitter.

Each time, Mattis maintained that it is inappropriate for former military men and administration officials to undermine the current commander in chief.

"Your personal library has thousands, thousands, 7,000 books in it. What does it say about a chief executive who does not read? I’m just speaking hypothetically," Mitchell asked.

"Andrea, I’m going to frustrate you here," Mattis replied.

"I think any time you have a leadership team, and that’s what the president and his Cabinet is, or in a corporation, it's the CEO and the board of directors, you have to have people with strengths and weaknesses," he said. "I know I'm not strong in all areas."

While he does not attack Trump in his new book, Call Sign Chaos, Mattis does criticize former Presidents George W. Bush and Barack Obama, as well as former Vice President Joe Biden.

Mattis added he does not think "one person can normalize anything. Right now, I’ve got a lot of faith in the American people to draw their own conclusions," saying he was first contracted to write the book in 2013.

"The U.S. military, since the time of George Washington, during Abraham Lincoln’s time, has dealt with this. George Marshall, Gen. Bradley said when generals retire their uniform, they should make sure they retire their tongue about politics. I stay with the military tradition all the way through, and the current political discussions are not something I want the U.S. military to be engaged in," he continued.

The retired Marine general said his conversations with Trump were open and frank, especially since Trump is famous for not holding back in his thoughts.

[Also read: Jim Mattis, Marine legend who became Trump's 'Mad Dog' Pentagon chief, prepares for media blitz]