Mattis: 'I don't know how I could have spoken more loudly' against Trump than by quitting

Former Defense Secretary James MattisJames Norman MattisLawmakers push back at Pentagon's possible Africa drawdown Overnight Defense: Book says Trump called military leaders 'dopes and babies' | House reinvites Pompeo for Iran hearing | Dems urge Esper to reject border wall funding request Trump called top military brass 'a bunch of dopes and babies' in 2017: book 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 BrzezinskiMika Emilie BrzezinskiBiden to CBS reporter: 'Why, why, why, why, why?' Brzezinski on Sanders-Warren: Someone isn't telling the truth Scarborough to GOP: 'What job is worth selling your political soul over?' 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.

Mattis resigned from the Defense Department in late 2018, shortly after President TrumpDonald John TrumpTrump denies telling Bolton Ukraine aid was tied to investigations Former senior Senate GOP aide says Republicans should call witnesses Title, release date revealed for Bolton memoir 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."