Mattis goes after Trump: The president 'tries to divide us'

While he did not cite any other officials by name, Mattis harshly criticized Pentagon leaders, including Defense Secretary Mark Esper, for their handling of the military response to race-related protests across the country.

Even as Esper reversed a decision to send home active-duty troops on alert to respond to unrest in the national capital region on Wednesday, Mattis argued for deploying the military at home only on "very rare occasions," and only at the request of state governors.

"We do not need to militarize our response to protests. We need to unite around a common purpose," he wrote.

He cited Esper's decision to pose in a "bizarre photo op" outside St. John's Episcopal Church in Washington, D.C., after Attorney General Bill Barr ordered the clearing of protesters on Monday night. Esper said on Wednesday that he hadn't known ahead of time that the photo op was happening.

Mattis called the decision to clear protesters in Lafayette Square an "abuse of executive authority" and said that Americans should "reject and hold accountable those in office who would make a mockery of our Constitution."

Mattis also urged the public to reject Esper's characterization of American cities as a "'battlespace' that our uniformed military is called upon to 'dominate,'" referring to the defense secretary's comments comments to governors on Monday. (Esper said on Wednesday that in retrospect he would have used different language "so as not to distract from the more important matters at hand or allow some to suggest that we are militarizing the issue.")

Mattis noted that when he joined the military, he took an oath to support the U.S. Constitution, and "never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens."

Militarizing the response to civil unrest, as Mattis proclaims "we witnessed in Washington, DC," sets up a "false conflict" between the military and civilian society, he said.

A White House spokesperson and multiple Pentagon spokespeople didn't immediately respond to a request for comment on Mattis' statement.