Former Chairman of the Joint Chiefs Adm. Michael Mullen has written a forceful op-ed in The Atlantic magazine, saying he was “sickened” by police and National Guard personnel forcibly removing peaceful protesters near the White House on Monday, while arguing that “our fellow citizens are not the enemy, and must never become so.”

Mullen, a retired four-star admiral who served as the top military advisor to Presidents George W. Bush and Barack Obama, wrote that the events of the past few weeks “made it impossible to remain silent.”

Though the 73-year-old admiral rarely speaks out politically, Mullen said President Trump had “laid bare his disdain for the rights of peaceful protest in this country” and risked politicizing the U.S. military by his actions. 

Late Monday, Trump gave a speech in which he claimed to be “an ally of all peaceful protesters” and threatened to send military personnel to the states to quell violence and riots. He later crossed the street to St. John's Church, as police and national guardsmen fired tear gas, rubber bullets, and used riot shields to remove peaceful protesters from gathering near The White House.

The president was accompanied by Defense Secretary Mark Esper and Gen. Mark Milley, the current chairman of the Joint Chiefs. Both men were criticized for taking part in the walk, though senior defense officials told reporters Tuesday they believed they were going to check on National Guard troops, not taking part in a presidential photo op.

“While no one should ever condone the violence, vandalism, and looting that has exploded across our city streets, neither should anyone lose sight of the larger and deeper concerns about institutional racism that have ignited this rage,” Mullen wrote.

Related: Trump threatens to have National Guard shoot protestors in Minneapolis

He added: “As a white man, I cannot claim perfect understanding of the fear and anger that African Americans feel today. But as someone who has been around for a while, I know enough — and I've seen enough — to understand that those feelings are real and that they are all too painfully founded.

Mullen usually keeps a low-profile, and is careful and measured in his words, according to a retired naval officer who served with him. “He wouldn't do this unless he was profoundly disgusted with the president's conduct,” the retired officer said on condition of anonymity in order to speak candidly.

“It is a mark of how extraordinary — how appalling — the times are, that Adm. Mike Mullen, former Chairman of the Joint Chiefs of Staff, has written this,” tweeted Eliot Cohen, a former State Department official who served with Mullen in the Bush administration. “It is, in a way, a smaller breach of a norm in order to protect much larger norms.”

In the op-ed, Mullen went on to call for all citizens to address the issue of police brutality and injustices against African Americans, and support and defend the first amendment right to peacefully assemble.

“Neither of these pursuits will be made easier or safer by an overly aggressive use of our military, active duty or National Guard,” Mullen continued. 

“The United States has a long and, to be fair, sometimes troubled history of using the armed forces to enforce domestic laws. The issue for us today is not whether this authority exists, but whether it will be wisely administered.”

Concluded Mullen, “This is not the time for stunts. This is the time for leadership.”

“Mullen is sending a message to Milley, which is remarkable,” the retired officer speculated of Mullen's purpose. “What you did was unsat. Resign now, in protest.”

You can read the full op-ed at The Atlantic.