Former defence secretary James Mattis broke his long silence against US President Donald Trump on Wednesday amid ongoing demonstrations against police brutality that have sparked a nationwide discussion on racism.
In a rare public comment, Gen Mattis condemned Mr Trump's militarised response to the protests.
Gen Mattis served under Mr Trump as defence secretary for two years from January 2017 before falling out with the President over Syria policy.
"I have watched this week's unfolding events, angry and appalled," he said.
Gen Mattis denounced Mr Trump's heavy-handed use of military force to quell protests and said the president was setting up a "false conflict" between the military and civilian society.
"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," Gen Mattis wrote in The Atlantic.
"We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership."
Gen Mattis objected to the force used to move back protesters so Mr Trump could visit St John's Church the day after it was damaged by fire during protests.
"We know that we are better than the abuse of executive authority that we witnessed in Lafayette Square. We must reject and hold accountable those in office who would make a mockery of our Constitution," Mattis said.
The comments from Gen Mattis struck a nerve with Mr Trump, who took to Twitter to call him "the world's most overrated General".
“I asked for his letter of resignation and felt great about it,” wrote Mr Trump. “I didn’t like his ‘leadership’ style or much else about him, and many others agree. Glad he is gone!”
Former president Barack Obama also weighed in on the nationwide discussion sparked by the killing of George Floyd in Minneapolis police custody.
During a virtual town hall meeting on Wednesday, Mr Obama called for police reform, urging the country’s mayors to review their police departments’ use of force policies and commit to reforms.
Addressing black Americans directly, Mr Obama said, “I want you to know that you matter, I want you to know that your lives matter, that your dreams matter," Mr Obama said.
Minnesota Attorney General Keith Ellison on Wednesday announced that Derek Chauvin, the former police officer who was arrested in the killing of George Floyd, is now charged with second-degree murder.
The charge was increased from third-degree murder last Friday.
Mr Chauvin knelt on Floyd's neck for more than eight minutes.
Three other former Minneapolis police officers – Tou Thao, Alexander Kueng and Thomas Lane, who were with Mr Chauvin at the time of the killing – were charged on Wednesday for aiding and abetting murder.