General Mark Milley, chairman of the Joint Chiefs of Staff, addresses troops at Bagram Airfield in Afghanistan on Nov. 28, 2019. (Erin Schaff/The New York Times)

Gen. Mark Milley, chairman of the Joint Chiefs of Staff, addresses troops in Afghanistan in 2019. President Donald Trump is expected to order the military to withdraw thousands of troops from Afghanistan, Iraq and Somalia by the time he leaves office.