Washington DC - General Mark Milley steps down on Friday after a tumultuous term as the top US military officer that saw him face repeated crises at home and abroad.

General CQ Brown will replace him, becoming the second Black officer after Colin Powell to serve as chairman of the Joint Chiefs of Staff, at a time when the Pentagon is headed by Lloyd Austin, the country's first Black secretary of defense.

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

Russia's invasion of Ukraine, the US withdrawal from Afghanistan, Donald Trump's refusal to accept his presidential election loss, and nationwide protests against police brutality are just some of the events that defined his four years as the top US officer.

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Milley is credited by his supporters with helping protect the country from Trump, but has been the target of intense criticism from the former president and his adherents.

His departure comes at a time when the US military – particularly its leadership – has been under repeated fire from conservative politicians and pundits, especially over the alleged imposition of "woke" policies on the armed forces.

General CQ Brown 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.

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 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.

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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.

Brown's nomination was one of more than 300 stalled by a dispute over Pentagon policies that assist troops who must travel to receive reproductive health care that is unavailable where they are stationed.

A single Republican senator who opposes those efforts has been preventing lawmakers from quickly approving senior military nominees in groups, and Brown was only confirmed in time through an individual vote on his nomination.