President Obama has become one of the nation’s most successful commanders in chief when it comes to at least one battlefield: the liberal agenda. In his final months in office, Mr. Obama is leaving a legacy of a more politically correct armed forces, with the top brass squarely behind him, save the Marine Corps. Perhaps not since Harry S. Truman’s landmark 1948 order to racially integrate the military has a president unleashed his own collective social revolution on the Pentagon with such success. In the process, the building also has become one of the administration’s loudest voices on battling climate...