The stunning rebuke from Milley on his last day as chairman of the Joint Chiefs of Staff illustrated the way the US military has been dragged into the increasingly volatile political arena since the Trump era.

At an elaborate military ceremony for his departure -- attended by Defense Secretary Lloyd Austin and President Joe Biden -- Milley did not name Trump, but there was no doubt about the target of his barb.

"We don't take an oath to a king, or queen, or a tyrant or a dictator," Milley said of American soldiers. "And we don't take an oath to a wannabe dictator."

Milley will be replaced as chairman of the Joint Chiefs of Staff by Air Force General Charles "CQ" Brown -- just the second African American to hold the top military job.

A barrel-chested army veteran of countless foreign deployments and high-level command posts, Milley served in uniform for four decades.

But he faced his highest-stakes challenge when Trump appointed him in 2019 to the career pinnacle of senior military advisor to the president.

During a four-year term -- continuing under Biden from 2021 -- Milley managed the harrowing exit of US troops from Afghanistan, special forces operations in Syria, and the enormous program to assist Ukraine's desperate fight against Russian invasion.

As chairman, "it was one crisis right after another," Milley told AFP last month.

Milley's years at the top, however, also saw the military involved in an unusual number of politicized controversies.

While the Biden administration has pressed for changes including renaming bases named after Confederate leaders in the Civil War, senior Republicans have repeatedly lashed out at what they claim are "woke" leftist policies in the ranks.

And that was nothing compared to the precarious situation Milley found himself in during the lead-up to and aftermath of the 2020 presidential election -- in which Trump, in an unprecedented political nightmare for the United States, refused to accept defeat.

At the height of tensions after Trump supporters stormed the US Capitol on January 6, 2021, Milley secretly called his Chinese counterpart to reassure Beijing that the United States remained "stable" and had no intention to attack China, according to the book "Peril" by Bob Woodward.

That revelation has caused lasting fury for Trump, who just this month wrote on his social media network that "in times gone by, the punishment would have been DEATH!" for Milley.

- Threats and attacks -

The barely veiled threat from Trump -- the clear frontrunner to be the Republican candidate in the 2024 presidential election -- prompted Milley to take "appropriate measures" for his safety, he told CBS News.

Biden lashed out Thursday during a speech at Trump's "heinous statements" and attacked the "deafening" silence from Trump's fellow Republicans on the threat.

Milley's replacement, chosen by Biden, will become the second Black top Joint Chiefs officer after Colin Powell. Austin, meanwhile, is the country's first Black secretary of defense.

Brown -- who officially takes the reins from Milley at midnight (0400 GMT) on Saturday -- was commissioned as a US Air Force officer in 1984 and is an experienced pilot with more than 3,000 flight hours, 130 of them in combat.

Brown, known to most as "CQ," even once survived ejecting from an F-16 during training over Florida.

He has commanded a fighter squadron and two fighter wings, as well as US air forces under the Central Command and Indo-Pacific Command, and served as chief of staff of the Air Force.

Following the 2020 murder of Black man George Floyd by a white police officer in Minnesota, Brown recorded an emotional video about his personal experiences, including with discrimination in the American military.

He said he felt pressure to "perform error-free," and worked "twice as hard" to prove wrong those who expected less of him because of his race.