Former Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE said Wednesday that he could not see a more forceful way to express his opposition to the Trump administration's policies than by leaving the Pentagon.

MSNBC "Morning Joe" co-host Mika Brzezinski Mika Emilie BrzezinskiMika Brzezinski: 'Super grossed out' by Trump speech attendees 'who put their lives at risk' Trump hurls insults at Harris, Ocasio-Cortez and other women Trump targets 'Complete psycho' Scarborough, 'ditzy airhead' Brzezinski while praising 'Fox & Friends' MORE questioned Mattis about when officials such as himself would begin to issue forceful condemnations of the Trump administration.

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"Well, I don't know how I could have spoken more loudly to where I stand [than] when I put in my letter of resignation, and quitting a job when I had not completed it, two years in," Mattis told Brzezinksi.

"This isn't about one man and the solution isn't going to be one person speaking out. It's going to be about the majority of Americans saying that's enough, we owe better to the next generation than what we're doing right now," Mattis added.

'This isn't about one man and the solution isn't going to be one person speaking out. It's going to be about the majority of Americans saying that's enough, we owe better to the next generation than what we're doing right now.' -- Fmr. Defense Secy. Jim Mattis pic.twitter.com/sHCugFUHdL — Morning Joe (@Morning_Joe) September 4, 2019

Mattis resigned from the Defense Department in late 2018, shortly after President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE announced a withdrawal of U.S. troops from Syria in a move that surprised top Defense officials and also resulted in the resignation of the top U.S. envoy to the multinational coalition formed to battle the Islamic State in Iraq and Syria.

Ahead of the release of his book, “Call Sign Chaos: Learning to Lead,” Mattis has given a series of media interviews about his time in the administration, though early reviews of his book suggest that it will not focus on his time working with Trump.

"I want people to understand why I couldn’t stay. I’ve been informed by four decades of experience, and I just couldn’t connect the dots anymore,” he told The Atlantic last week.

“If you leave an administration, you owe some silence," he added at the time. "When you leave an administration over clear policy differences, you need to give the people who are still there as much opportunity as possible to defend the country."