As the civil unrest caused by the death of an African-American man by a police officer continues to rock the country, former Secretary of Defense James Mattis penned a missive against President Trump.

“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 wrote in his missive, which was published by The Atlantic.

“I have watched this week’s unfolding events, angry and appalled,” Mattis continued. “The words ‘Equal Justice Under Law’ are carved in the pediment of the United States Supreme Court. This is precisely what protesters are rightly demanding. It is a wholesome and unifying demand—one that all of us should be able to get behind. We must not be distracted by a small number of lawbreakers. The protests are defined by tens of thousands of people of conscience who are insisting that we live up to our values—our values as people and our values as a nation. We must reject and hold accountable those in office who would make a mockery of our Constitution.”

Before becoming the SECDEF, Mattis had a very successful career as an officer in the Marine Corps, reaching the rank of General. Known for his wit and no-nonsense approach, Mattis was quite popular with the troops during and after he left the Service. Trump appointed Mattis as SECDEF when he was first elected in 2016. Mattis, however, resigned in 2018 after disagreements over policy with the White House.

Trump responded to the letter with a series of Tweets. “Probably the only thing Barack Obama & I have in common is that we both had the honor of firing Jim Mattis, the world’s most overrated General,” tweeted the President.

Meanwhile, the incumbent SECDEF, Mark Esper, publically disagreed with Trump’s warning to deploy federal troops in order to deal with the civil unrest.

Read James Mattis' letter to the Pentagon after being sworn in as SECDEF

Read Next: Read James Mattis' letter to the Pentagon after being sworn in as SECDEF

During a press conference, Esper said that “I’ve always believed and continue to believe that the National Guard is best suited for performing domestic support to civil authorities in these situations in support of local law enforcement.”

Esper added that the current situation doesn’t warrant the deployment of Active Duty troops nor the invocation of the Insurrection Act, which would allow the President to order federal troops on U.S. soil.

“We are not in one of those situations now. I do not support invoking the Insurrection Act,” added the SECDEF.

The deployment of federal line troops should be the last resort. The Active Duty military is combat-oriented, particularly units such as the 82nd Airborne Division that was put on alert to deal with the riots in D.C. It should not be asked to perform police duties unless there is no other option. The National Guard is more than capable to deal with the current situation in conjunction with the local and state law enforcement departments. Federal Intelligence, Surveillance, and Reconnaissance (ISR) assets, however, could help with the policing as they will allow local and state officials to make better-informed decisions.

In a letter sent out to all units, Chairman of the Joint Chiefs of Staff General Mark Milley emphasized that all service members have sworn an oath to the American Constitution. He also highlighted that all Americans have the right to freedom of speech and peaceful assembly.

“We all committed our lives to the idea that is America — we will stay true to that oath and to the American people,” added Gen. Milley.