WASHINGTON (AP) — In his first eight months as chairman of the Joint Chiefs of Staff, Gen. Mark Milley carefully crafted a low-key public profile. He knew that splashy and sassy were unlikely to endear him to his boss, President Donald Trump.

Then "the walk" happened.

Milley, in his camouflage battle dress uniform, strolled with Trump and a presidential entourage across Lafayette Square on June 1 to be positioned in front of a church, where Trump held up a Bible for photographers. Critics immediately hit Milley, the nation's top military officer, for appearing to be a political pawn. On Thursday, he finally spoke out.

"It was a mistake," he said — simple words that thrust the square-jawed general into the public spotlight like never before. That might not sit well with a president who dislikes any hint of criticism of his staged events, let alone criticism from those serving under him.

Milley's words drew praise from some members of Congress, including Trump ally Sen. Lindsey Graham of South Carolina, but added a new layer of tension between the Pentagon and the White House. That tension burst into public last week with Defense Secretary Mark Esper openly breaking with Trump on the use of federal troops to quell protests.

Milley's willingness to admit he erred reflects a personal commitment to a principle deeply rooted in American military tradition: that members of the military are apolitical, sworn to defend the Constitution, not a president. Civilians are supposed to control the military, but not for personal political gain.

That commitment had been questioned after the walk. Sen. Tammy Duckworth, D-Ill., an Iraq War veteran, said seeing Milley and Esper "walking like lapdogs" behind Trump sends "a horrifying message to our troops — including our black and brown troops — that our military's leaders will not protect them from unlawful orders."

Former senior military leaders also weighed in, though in many cases didn't mention Milley by name. Jim Mattis, the retired Marine general who was Trump's first defense chief, called it "a bizarre photo op for the elected commander in chief, with military leadership standing alongside."

But it was Milley, along with other senior officials, who was urging Trump not to carry out his threat to deploy active-duty troops in D.C. As part of that effort, he spent time at the main security operations center, working to ensure that National Guard members were in the right places and had what they needed to protect the city, so active-duty forces would not be required.

Striding along the streets talking with Guard members, he was a formidable figure with his piercing look, close-cropped gray hair, bushy eyebrows and imposing size. He talked openly about the need for troops to enable the peaceful protests while also preventing violence.

Military generals are known for their egos, and Milley is no exception. Reserved is not a word one would use to describe the Massachusetts native. He is forceful, given to bouts of temper but quick with a laugh and a sharp retort.

A student of military history, Milley peppers his conversations with references to lessons learned during America's wars, the details of which he can recite instantly. He is given to regaling visitors to his official residence above Arlington National Cemetery with stories about the history of the house and its previous occupants.

Milley's father, Alexander, served as a Navy corpsman with the 4th Marine Division during World War II and fought with them at the Battle of Iwo Jima. In a speech in January, Milley recalled walking the beach where his father landed and said he "began to imagine what hell on earth was like and that was Iwo Jima." Reflecting on the costs of war, he took that moment to repeat his conviction that the military's job is to preserve the Constitution and prevent such war from happening again.

After four decades in the military, Milley was a somewhat surprising pick by Trump to be Joint Chiefs chairman, a position that is the pinnacle of any military career. He had been serving as chief of staff of the Army when Mattis was ready in 2018 to recommend to the White House who should succeed Gen. Joseph Dunford as chairman. Milley was notably not his choice.

Trump, however, disregarded Mattis' advice and chose Milley, saying, "He's a great gentleman, he's a great patriot, he's a great soldier."

Two weeks later Mattis submitted his resignation, a move widely attributed to his disagreement with Trump over pulling out of Syria but also linked to his disappointment at Trump overruling him on Milley, who became chairman on Oct. 1, 2019. He commands no troops but is the principal military adviser to the president, the secretary of defense and the National Security Council.

Milley is known in the military as a charismatic leader who commanded troops during several tours in Iraq and Afghanistan. He hasn't been afraid to offer candid and sometimes blunt assessments, including to Congress. In 2017, he admonished the House Armed Services Committee for being slow to approve a defense budget, slamming the inaction as "professional malpractice."

In his three years as Army chief of staff, he helped shepherd the groundbreaking move of women into front-line infantry and other combat positions. He also played a role in one of the Army's more contentious criminal cases, making the early decision to charge former Army Sgt. Bowe Bergdahl with desertion and misbehavior before the enemy. Bergdahl abandoned his post in Afghanistan and was held captive by the Taliban. He was eventually found guilty and dishonorably discharged.

A native of Winchester and a vocal fan of the Red Sox and other Boston teams, Milley received his Army commission in 1980 from Princeton University, where he earned a bachelor's degree in political science. He has two master's degrees.

An infantry officer by training, he also commanded Special Forces units in a career that has included deployments in the invasion of Panama in 1989 and the multinational mission in Bosnia-Herzegovina to implement the Dayton Peace Accords.