GUANTANAMO BAY NAVY BASE, Cuba — Earlier this month, in a drill shrouded in secrecy, prison guards practiced for something that hasn’t actually happened at Guantanamo in a decade. They rehearsed receiving a new war-on-terror detainee. Medical evaluation? Check. Notification of the International Red Cross? Check. Assignment to a cell? Check. Security and more security. Gone are the iconic orange uniforms that made Camp X-Ray infamous. The man who played the role of new captive wore white. Navy Rear Adm. John Ring, the prison commander, says he hasn’t gotten any word that new prisoners are coming. But since President Donald Trump signed an order keeping the prison open, Ring’s staff is now preparing for what spokeswoman Navy Cmdr. Anne Leanos calls an “enduring mission.” “I have not been told we’re getting new people. I have no order to receive new people. I’ve been asked some hypothetical questions about capacities and things like that, but we are not imminently expecting anybody,” Ring told reporters in early June. Guantanamo today has 40 prisoners and a staff of 1,800 troops and civilians. With the maximum-security Cam...