In June 2020, former Secretary of Defense James Mattis wrote an op-ed titled “In Union there is Strength” in which he attacked Trump’s handling of ongoing protests in the wake of George Floyd’s death at the hands of police.

Mattis specifically referenced Trump and what he considered to be a failure of leadership:

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. We are wit­ness­ing the con­se­quences of three years of this de­lib­er­ate ef­fort. We are wit­ness­ing the con­se­quences of three years with­out ma­ture lead­er­ship. We can unite with­out him, draw­ing on the strengths in­her­ent in our civil so­ci­ety. …We must re­ject and hold ac­count­able those in of­fice who would make a mock­ery of our Con­sti­tu­tion. At the same time, we must re­mem­ber Lin­coln’s “bet­ter an­gels,” and lis­ten to them, as we work to unite.

Mattis also commented on Trump’s handling of ongoing protests in the wake of George Floyd’s death at the hands of police:

At home, we should use our military only when requested to do so, on very rare occasions, by state governors. Militarizing our response, as we witnessed in Washington, D.C., sets up a conflict—a false conflict—between the military and civilian society.

Trump responded via tweet, calling Mattis “the world’s most overrated general.”

Mattis, a retired four-star Marine Corps general, resigned as SecDef in 2018 in protest to Trump’s Syrian policy and withdrawal of US troops from the area.

The Atlantic – James Mattis Denounces President Trump, Describes Him as a Threat to the Constitution
The New York Times – Mattis Accuses Trump of Dividing the Nation in a Time of Crisis