Updated at 6 p.m. ET

Ten days after leaving the White House with President Trump and walking with him across a park that had been forcibly cleared of protesters, the nation's most senior military officer is calling that excursion "a mistake."

"I should not have been there," Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, declared Thursday in a video message to graduating officers at the National Defense University. "My presence in that moment and in that environment created a perception of the military involved in domestic politics."

Milley has drawn pointed criticism from retired military officers not only for accompanying the president in what turned out to be a staged photo-op in front of a boarded-up St. John's Church, but for having done so dressed in baggy battle fatigues. The four-star Army general appeared to express regret for that as well.

"We who wear the cloth of our nation come from the people of our nation, and we must hold dear the principle of an apolitical military that is so deeply rooted in the very essence of our republic," Milley said in the commencement address. "And this is not easy."

Milley, whom Trump chose last year to hold the military's highest rank, also admonished the graduating class of commissioned officers to embrace the U.S. Constitution.

"The freedoms guaranteed to us in the Constitution allow people to demand change, just as the peaceful protesters are doing all across the country," he said. "As you graduate today, reflect on what you have witnessed over the past two and a half weeks — what it means to all of us as Americans, and what it means to you and I, as leaders."

Both Milley and Defense Secretary Mark Esper, who also accompanied Trump in his walk across the cleared Lafayette Square, have said they were not aware where Trump was going when they fell in step behind him, and that they thought they were leaving the White House to thank National Guard troops stationed nearby. Aides have also said Milley was dressed in fatigues because he was headed to an FBI operations center.

That excursion across Lafayette Square has been harshly criticized by Trump's first defense secretary, retired Marine Gen. James Mattis, who resigned from that post in December 2018.

"When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution," Mattis wrote in a statement published last week in The Atlantic. "Never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens — much less to provide a bizarre photo op for the elected commander-in-chief, with military leadership standing alongside."

Similar strong criticism of the crackdown on protesters that cleared the way for Trump and his entourage is being leveled by Robert Gates, who was defense secretary in both the George W. Bush and Barack Obama administrations.

"I think it's just a reality that this president has not seen it as one of his priorities to try and bring people together, either in general or in response to terrible events, whether it's the pandemic or the response to George Floyd's death," Gates told NPR's Morning Edition. "It's hard to disagree that this president is a divider. He's not the only one in American history, but he certainly has taken it, I think, to a level that we haven't seen in quite a long time."

In his remarks to the National Defense University graduating class, Milley offered a piece of advice that appears to stem from a lesson he learned from the Lafayette Square episode.

"Always maintain a keen sense of situational awareness," the 61-year-old general said as he stood before a backdrop of military flags. "As senior leaders, everything you do will be closely watched, and I am not immune."

Unlike Esper, Milley did not appear in photos of Trump holding a Bible in front of St. John's Church. But many images were taken of Milley trailing behind Trump in a place that had been cleared of protesters only minutes earlier.

"As a commissioned uniformed officer, it was a mistake that I learned from," Milley told the graduates. "And I sincerely hope we all can learn from it."

Copyright 2020 NPR. To see more, visit https://www.npr.org.