" 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," Mattis, who resigned as the Pentagon chief in December 2018 in protest against Trump's Syria policy, said on Wednesday in an article carried by the Atlantic magazine.

 

"We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership," he was quoted as saying in the article by Xinhua news agency.

 

"We can unite without him, drawing on the strengths inherent in our civil society.

 

"This will not be easy, as the past few days have shown, but we owe it to our fellow citizens; to past generations that bled to defend our promise; and to our children," he said.

 

Mattis' excoriation came as Trump threatened to send in active-duty military forces to quell the ongoing protests against police brutality and racial discrimination that have spread to over 300 US cities and towns following the killing of Floyd, an unarmed black man in Minneapolis, by white police officers.

 

"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," Mattis added.

 

Meanwhile, incumbent Defence Secretary Mark Esper has also openly opposed Trump's suggestion to use the military to quell the disturbances.

 

Esper said on Wednesday that the nation was not in a situation that would legally allow Trump to call out the troops for domestic operations.

 

"The option to use active-duty forces in a law-enforcement role should only be used as a matter of last resort and only in the most urgent and dire of situations.

 

"We are not in one of those situations now. I do not support invoking the Insurrection Act," he added.