WASHINGTON — When former Gen. Colin Powell rebuked President Donald Trump on Sunday, saying he had “drifted away from the Constitution,” he became the fourth former chairman of the Joint Chiefs of Staff to speak out against Trump’s desire to use active-duty soldiers to confront protestors.

According to several military experts contacted by NBC News, it is unprecedented for so many former top military officials to criticize a sitting commander-in-chief harshly and publicly.

“It’s unusual for any flag officers, or indeed any former officers, to speak out against the commander-in-chief,” said David Segal, the founding director of the Center for Research on Military Organization at the University of Maryland, explaining that officers generally try to remain apolitical. “In the 50 years I’ve been studying the military, I can’t think of a precedent.”

“And the fact that there hasn’t been any pushback, I think is even more telling,” he said. “I don’t see any controversy from the ranks or from society.”

Former White House and State Department official Heather Hurlburt agreed.

She said that in the past when there have been heated public debates about how the U.S. military should be used, they have not provoked the same response from flag officers. She cited the large protests against the Vietnam War, the transition from the draft to an all-volunteer force, the Reagan administration’s actions in Central America, the repeal of Don’t Ask Don’t Tell, and the decision to allow women to serve in combat.

Hurlburt, now at the New America Foundation, said that when such controversies occur, former chairs definitely make their views known in private. However, she said, “It’s rare for any to speak publicly and in none of the instances above did four speak out.”

Powell’s statement on Sunday followed criticism of Trump from more than a half-dozen former flag officers, including Ret. Adm. Bill McRaven, commander of the raid that killed Osama bin Laden; Trump’s own former Defense Secretary, Ret. Gen. James Mattis; and three other former chairmen of the Joint Chiefs. On Tuesday, Ret. Adm. Mike Mullen called Trump’s desire to use active-duty troops to suppress protests “sickening.” Ret. Gen. Martin Dempsey called it “dangerous” and “very troubling” in an interview with NPR on Thursday. Ret. Gen. Richard Myers said the tear-gassing of protestors outside the White House Monday night filled him with “just absolute sadness,” and said he was glad he was not advising Trump.

Segal sees a change in the military and thinks it is now being drawn into politics more than in previous generations “I think it clearly is a break from the past,” he said. “The question is whether it portends a change in direction or a reflection of this particular moment in time and this particular president.”