General Mark A Milley, chairman of the Joint Chiefs of Staff, who appeared behind Mr Trump in combat fatigues on that day, said: “I should not have been there.”
Authorities used teargas and rubber bullets to clear the park outside the White House of peaceful protesters who has gathered in the wake of the killing of George Floyd by police officers in Minneapolis.
“My presence in that moment and in that environment created a perception of the military involved in domestic politics,” Gen Milley continued.
“As a commissioned uniformed officer, it was a mistake that I have learned from,” he said in a prerecorded video commencement address to National Defence University.
Acknowledging that his presence raised questions about the role of the military in civil society, he continued: “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.”
“And this is not easy. It takes time and work and effort. But it may be the most important thing each and every one of us does every single day,” he added.
Gen Milley also said that he too is angry about the “senseless and brutal killing of George Floyd”.
After the clearing of the park, the president and a group of White House staff and officials walked out for Mr Trump to pose for pictures with a Bible in front of St John’s Church, in a widely condemned stunt.
Gen Milley’s first comments since the incident will likely anger the White House as the president takes an increasingly tough line against the growing protest movement.
The remarks also further amplify the deepening split between the Trump administration and the military.
The New York Times reports that Gen Milley has agonised about his appearance behind the president that day after it appeared that the military approved of the hardline tactics used to clear the park of protesters.
In the days after the photo op, the general reportedly told Mr Trump that he was angered by what unfolded that afternoon, and Pentagon officials say he believed that the group had left the White House to review National Guard troops in the square.
Both Gen Milley and defence secretary Mark Esper also opposed the president’s wish to have active-duty troops on the streets to counter the protest movement, arguing that it was largely peaceful and should be handled by local law enforcement.
A number of military officials have ruled out using troops against protesting American citizens under the 1807 Insurrection Act.
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