Since resigning as Secretary of Defense, James Mattis has kept his opinions about Donald Trump to himself. But everyone has a breaking point, and watching television this week, Mattis found his.

The Washington Post reports:

For former defense secretary Jim Mattis, it was the last straw: the sight of the chairman of the Joint Chiefs of Staff, walking the streets of downtown Washington in battle-ready camouflage amid a show of brute federal force.

Smoke was still rising from Lafayette Square, where authorities had just used pepper spray and smoke canisters to disperse a group of largely peaceful protesters, when Gen. Mark A. Milley, along with Defense Secretary Mark T. Esper, joined President Trump Monday evening as he strolled to a nearby church to pose for cameras with a Bible.

According to the Post, Mattis believed the appearance of two top military leaders condoned the use of force against the peaceful protesters. “Mattis fumed that the president was using the leaders who replaced him at the Defense Department to further divide the nation, according to four people familiar with his thinking.

He was apparently most upset that Gen. Milley was wearing combat fatigues for the occasion.

“The military was never set up to prop up anyone’s political agenda, and I think that really pissed him off, when he saw that,” said Carlton Kent, a retired Marine sergeant major who advised Mattis in Iraq. “He never wanted them to be in a compromising situation.”