After long refusing to explicitly criticize a sitting president, former Defense Secretary Jim Mattis accused President Donald Trump on Wednesday of trying to divide America and roundly denounced a militarization of the US response to civil unrest.

The remarks by Mattis, an influential retired Marine general who resigned over policy differences in 2018, are the strongest to date by a former Pentagon leader over Trump’s response to the killing of George Floyd, an African-American, while in Minneapolis police custody.

“Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try,” Mattis, who resigned as Trump’s defense secretary in 2018, wrote in a statement published by The Atlantic.

“Instead he tries to divide us. We are witnessing the consequences of three years of this deliberate effort.”

Trump has turned to militaristic rhetoric in the wake of Floyd’s killing by a white police officer, who knelt on his neck for almost nine minutes in Minneapolis last week.

On Monday Trump threatened to send active duty US troops to stamp out civil unrest gripping several cities, even against the wishes of state governors — alarming current and former military officials, who fear dissent in the ranks and lasting damage to US armed forces itself, one of America’s most revered and well-funded institutions.

“Militarizing our response, as we witnessed in Washington, D.C., sets up a conflict — a false conflict — between the military and civilian society,” Mattis wrote.

Trump responded by Twitter by calling Mattis “the world’s most overrated General!”

“I didn’t like his “leadership” style or much else about him, and many others agree. Glad he is gone!” Trump wrote.

A prominent figure in military circles, Mattis’ strong words could inspire others in uniform and veterans to speak out and are particularly surprising given his extreme reluctance to criticize Trump in scores of interviews and appearances since he left office over policy differences with the US President.

His comments follow denunciations by other retired top brass, including Navy admiral Mike Mullen and retired Army general Martin Dempsey, both former chairmen of the Joint Chiefs of Staff.