Trump initially reacted to Mattis' resignation gracefully, tweeting that the defense chief and retired Marine general would be "retiring, with distinction, at the end of February", echoing Mattis' resignation letter.
Bernstein, who helped uncover the Watergate scandal in the early 1970s, made his remark on CNN's Reliable Sources show on Sunday, three days after Mattis resigned.
The announcement appeared to catch Pentagon officials off guard. That drew a critical tweet from Trump on Monday.
"I've encouraged him to find people who have some overlap with him but don't see the world in lockstep with him", Mulvaney said. "Some thought I shouldn't, I thought I should".
Army Col. Rob Manning, a Pentagon spokesman, issued a statement Sunday that did not directly address the personnel changes. "The department remains focused on national security".
Yuri Gripas/ReutersDefense Secretary James Mattis.
President Donald Trump initially heralded Mattis, saying he would leave at the end of February.
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He's paid now to give his opinion and of course he should do. "Paul loves playing for this club". The World Cup victor only played 15 minutes in the Portuguese's final three games in charge.
He also served as the deputy assistant secretary of state for Iraq and Iran, and worked under Republican George W. Bush as a senior official on Iraq and Afghanistan. Mattis said that Trump deserved to have a defense secretary more aligned with his views.
The scope of Mattis' divergence with Trump went far beyond just the USA military missions in Syria and Afghanistan.
Trump was not shy about sharing his frustrations publicly.
Then the president took a shot directly at Mattis, with a bit of revisionist history. That makes him an unusual pick for secretary of defense.
Outgoing Defense Secretary Jim Mattis sent a holiday message to United States service members in which he commended them and noted their sacrifice, yet also said, "storm clouds loom". Interesting relationship-but I also gave all of the resources that he never really had. "Mattis believes that allies are an important cornerstone of USA national security". Job one for the Turkish Strong Man, of course, is not going after ISIS; it is wiping out our allies the Syrian Kurds-who Erdogan views as mortal enemies. "Grandstander? The Fake News is making such a big deal about this nothing event!"
Trump, turning to Twitter after days of criticism over his decision to pull U.S. troops out of Syria, denied charges that he was rejecting allies. During a lunch with conservative lawmakers Saturday at the White House, Trump discussed his options.
Shanahan's biography on the Pentagon's website does not list military experience for the longtime Boeing executive.
Jim Mattis - who checks in with their mother nearly daily, Tom Mattis said - had no plans to return home from Christmas, according to the elder Mattis, hoping instead to visit troops in the Middle East.