evidence: President Richard Nixon greets troops in South Vietnam in July 1969.David Turnley/Corbis/Getty Images [...] President Donald Trump and first lady Melania Trump greet members of the military as they arrive for a hangar rally at Al Asad Air Base in Iraq in December 2018.Andrew Harnik/AP Trump serves food to US troops during a surprise visit to Afghanistan in November 2019.Tom Brenner/Reuters