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.

In Mattis’s eyes, the appearance of the two top military leaders appeared to condone an unprovoked use of force. The nonpartisan military that Mattis had served for nearly five decades was being featured as decoration for a photo op, and 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 especially upset to see Milley — whom Mattis believed had sought to curry favor with Trump when he was defense secretary — appear in his Army combat uniform at a peaceful demonstration. That jarring image highlighted the military’s involvement in a heavy-handed crackdown on civilians…

“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.”