At fortified White House, Trump under verbal assault from military's top ranks

Fortified by troops in uniform, foreboding fencing and police in riot gear, President Donald Trump remained protected from largely peaceful protesters this week -- but not from the ire of his predecessors and the nation's top military figures.

While Trump has always embraced his outsider status among the elite president’s club, his defensive tone and calls to “dominate” demonstrators have stood in sharp contrast with rare statements released by previous presidents calling for unity and reflecting on persistent racial injustices.

In what amounted to the most direct rebuke of the president from a former member of his Cabinet, Trump's first and former defense secretary James Mattis on Wednesday denounced the commander in chief as a threat to the Constitution, saying he’s been “angry and appalled” in watching his handling of the protests that have followed George Floyd’s death.

"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 wrote in an essay in The Atlantic. "Instead he tries to divide us."

It wasn’t just his former, but also his current, secretary of defense who broke with the president.

Defense Secretary Mark Esper said that he did not think active-duty military troops should be dispatched to American cities to quell unrest, after Trump threatened to do so.

His break with the president was not well received at the White House, where the press secretary, Kayleigh McEnany, made clear the option remained on the table for the president to invoke a 213-year-old law, the Insurrection Act, which would allow him to do so.

"This president has one singular aim, and it is protecting America's streets," McEnany told reporters Wednesday.

Past presidents weigh in

Trump has also found himself increasingly at odds with former presidents.

Former President Barack Obama expressed solidarity with peaceful protesters, saying their cause represented an "an incredible opportunity” for the country to confront the issues of systemic racism.

"They offer an opportunity for us to work together to tackle them, to take them on, to change America and make it live up to its highest ideals," Obama said during a virtual town hall Wednesday. "Part of what’s made me so hopeful is the fact that so many young people have been galvanized and activated and motivated and mobilized. Because historically, so much of the progress that we've made in our society has been because of young people."

Speaking directly to young people of color, he said, "I want you to know that your lives matter. Your dreams matter."

Former President George W. Bush said he and former first lady Laura Bush “have resisted the urge to speak out, because this is not the time for us to lecture. It is time for us to listen.”

“It is a strength when protesters, protected by responsible law enforcement, march for a better future,” Bush said. “Those who set out to silence those voices do not understand the meaning of America — or how it becomes a better place.”

Former President Bill Clinton said it’s the time for the country to ask tough questions and reflect on persistent racism but that “we can’t honestly answer these questions in the divide and conquer, us vs. them, shift the blame and shirk the responsibility world we’re living in."

“People with power should go first—answer the questions, expand who’s ‘us’ and shrink who’s ‘them,’ accept some blame, and assume more responsibility. But the rest of us have to answer these questions too,” Clinton said.

Former President Jimmy Carter said in a statement that he and former first lady Rosalynn Carter are "pained by the tragic racial injustices and consequent backlash across our nation in recent weeks."

"We need a government as good as its people, and we are better than this," the oldest living president said in a statement.

Mattis's 'symbolic barrier'

While rare -- and perhaps unprecedented -- for former military leaders to so publicly and forcefully criticize the commander in chief, Trump's flaunting of the Insurrection Act "was sufficiently disturbing that General Mattis chose to break his silence," Christine Wormuth, who served as served as under secretary of defense for policy from 2014 to 2016, told ABC News.

The essay was the "inevitable conclusion" of the increasingly politicization of the military under the Trump administration and was meant to give a clear reminder to the military that they swore an oath to the Constitution, she said.

"It sends a strong message to members of the military to be reminded of what's important and to do what they think is right," Wormuth, now at the RAND Corporation, said.

Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, similarly sent an unusual message to the leaders of the different branches of the military that said members of the armed forces swore an oath to the Constitution and its protections for freedom of speech and assembly. The letter served as "cover" for the service leaders to "communicate their feelings" as well, Wormuth said.

Senate Minority Leader Chuck Schumer said Mattis's words sent a "shot across the bow" to the country's military leaders and could prove problematic for Trump.

"Mattis put a big symbolic barrier in his way and bolstered, I think, the instincts of some of the good people in the military who say we can’t be used like this," Schumer said in an interview with MSNBC.

Sen. Lisa Murkowski, R-Alaska, broke with most of her GOP colleagues on Capitol Hill by embracing Mattis' statement as "true, and honest, and necessary and overdue."

"I have been struggling for the right words, and I was encouraged a couple of nights ago when I was able to read what President Bush had written," she told reporters Thursday. "And I found that to be empowering for me as one leader.

"But then when I saw General Mattis’ comments yesterday, I felt like perhaps we are getting to a point where we can be more honest with the concerns that we might hold internally," she added. "And have the courage of our own convictions to speak up."

Another Republican senator who has been more willing to criticize Trump than his colleagues, Mitt Romney of Utah, called Mattis's words "stunning and powerful."

"General Mattis is a man of extraordinary sacrifice," Romney said. "He's an American patriot. He's an individual whose judgment I respect, and I think the world of him. If I ever had to choose somebody to be in a foxhole with -- it would be with a General Mattis. What a wonderful, wonderful man.”

Amid the onslaught, stalwart Trump ally Sen. Lindsey Graham, R-S.C., told Fox News that Mattis did not "understand" that people are trying to hurt Trump.

"The one thing I would tell General Mattis is that you don't quite understand that from the time President Trump wakes up till he goes to bed there's an effort to destroy his presidency," Graham said.

Like Trump, he pointed his finger at the press.

"To General Mattis, I think you're missing something here, my friend," Graham said. "You're missing the fact that the liberal media has taken every event in the last three and half years and laid it at the president's feet. I'm not saying he's blameless, but I am saying that you're buying into a narrative that I think is quite frankly unfair."

ABC News' Allison Pecorin and Trish Turner contributed reporting.