Rice University history professor Douglas Brinkley told CNN’s Jim Sciutto on Friday that he’s never seen so many former military leaders all come out en masse to question a president’s fitness for office.

While discussing the latest remarks from Ret. Gen. John Kelly, who previously served as Trump’s chief of staff, Brinkley said it was surprising to see so many former military leaders speak out against the president.

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“What is unique here, these are people that know Donald Trump well, saying he’s unfit for command,” Brinkley said. “[Ret. Adm. William McRaven] is perhaps the greatest Navy SEAL in U.S. history, and he’s saying I feel embarrassed for our children that we’re allowing a president like Donald Trump to run shotgun over the U.S. armed forces.”

Sciutto then noted that these retired military leaders aren’t just saying that Trump is erratic and temperamental, but are also saying that he lacks the moral character to lead the country.

“We have never had something like this, they’re saying the character of President Trump is unfit, that he’s a moral degenerate,” he said. “Gen. Kelly who knows him well being White House chief of staff, famously said, ‘If I leave… this president is going to be impeached.’ Meaning he doesn’t understand limits of executive power, he’s going to abuse his power.”

Watch the video below.