WASHINGTON Top U.S. general Mark Milley will retire on Friday after a four-year tenure that saw successes like the killing of ISIS head Abu Bakr al Baghdadi and helping Ukraine to defend against Russia's invasion, but also included the chaotic withdrawal from Afghanistan and a rocky relationship with former President Donald Trump.

Milley will hand over command to Air Force chief General Charles Q. Brown, who will be only the second Black officer to become chair of the Joint Chiefs of Staff, after Colin Powell two decades ago.

Milley took the reins in 2019 after being nominated by Trump, but soon found himself having to balance the need to maintain his relationship with the former president without appearing to be political.

In 2020, he publicly apologized for joining Trump as he walked from the White House to a nearby church for a photo opportunity after authorities cleared the way of protesters using tear gas and rubber bullets.

Milley said on Wednesday he would take measures to protect his family after Trump suggested he had colluded with China in an act he said would have once warranted death.

Brown is a self-described introvert whose public persona contrasts sharply with the outgoing Milley, a loquacious Boston native.

When Brown, a former fighter pilot with experience in the Pacific, takes command, Black Americans will hold the top two positions at the Pentagon for the first time - a major milestone for an institution that is diverse in its lower ranks but largely white and male at the top.

The move follows President Joe Biden's appointment of Lloyd Austin to become the first Black U.S. secretary of defense, the top civilian position at the Pentagon.

(Reporting by Phil Stewart and Idrees Ali; Editing by Lisa Shumaker)