General Mark Milley retires, takes a parting shot at ex-President Donald Trump.
Milley's ceremony highlights the military's political role during the Trump era.
Without naming Trump, he criticizes service to a "wannabe dictator."
Air Force General Charles "CQ" Brown becomes the second African American in his role.
Milley's 40-year career includes handling Afghan withdrawal and supporting Ukraine.
His tenure saw the military in political controversies.
Milley privately reassured China during post-election chaos, angering Trump.
Trump's threats led to increased security measures for Milley.
Biden condemns Trump's remarks and Republican silence.
Brown, an experienced pilot, succeeds Milley.