I think the editors of the Wall Street Journal have this about right:
Former Defense Secretary Jim Mattis’s denunciation of President Trump on Wednesday isn’t surprising, but it still looks like an important political moment. Mr. Trump’s polarizing and hyper-personal governance is catching up with him, as we and so many others warned.
Mr. Mattis, the former four-star Marine General, is a man of accomplishment and dedication to country. He made the decision to join Mr. Trump’s Administration despite the misgivings he must have had about the President’s foreign-policy views. He served loyally until he resigned on Dec. 20, 2018 after Mr. Trump’s abrupt decision to withdraw troops from the Syrian border with Turkey after a telephone call with Turkey’s President.
But as he so often has, Mr. Trump couldn’t resist kicking Mr. Mattis as he was going out the door. His initial tweets were supportive, but within two days he was criticizing Mr. Mattis for not helping enough to dun allies for more cash for U.S. foreign deployments. “General Mattis did not see this as a problem. I DO, and it is being fixed!” he tweeted.
He told a cabinet meeting that “I wish him well. I hope he does well. But, as you know, President Obama fired him and essentially so did I.” Mr. Mattis said in his resignation letter he’d stay until Feb. 28, but Mr. Trump ordered him out on Jan. 1.
In his statement to the Atlantic, Mr. Mattis denounced in particular Mr. Trump’s threat this week to order the military to restore order amid riots in U.S. cities. He said this threatens the Constitution, which is overwrought given that George H.W. Bush and other Presidents have done this. Mr. Mattis also undersold the significant harm that riots have done in many cities (see nearby).
But the general’s real motivation here is to tell the public that Mr. Trump lacks the character to be President and should be defeated in November. “Donald Trump is the first president in my lifetime who does not try to unite the American people—does not even pretend to try. Instead he tries to divide us,” Mr. Mattis said. “We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership.”
There is a reason that presidents of the last 60 or more years have followed one of two paradigms: either they have been micro-managers like Clinton or Carter, “policy wonks” attempting to run every detail of the federal government themselves, or staff managers like Eisenhower or Reagan who appointed good subordinates, established the objectives, let the subordinates work out the details for themselves, and supported them. The reason is that those approaches can be made to work. What President Trump has been doing cannot, particularly when so much of the appointed and permanent civil bureaucracy disagrees with you or even despises you.