The Defense Department’s fiscal 2019 budget had already carved out funds for fighting the Islamic State in Iraq and Syria, continuing the endless war in Afghanistan and preparing for a potential conflict with a foreign nation, such as China, Russia, North Korea or Iran. There has been no money set aside to combat the men, women and children who are bound for the American border, many of them fleeing violence or corruption, nearly all seeking better lives. The troops are tasked with the same types of logistical, support and even clerical jobs that National Guard soldiers sent to the border earlier this year are already doing. The military’s morale issue is almost as worrisome. The deployment orders last until Dec. 15, meaning the troops will be on the border over Thanksgiving. They will have little to do beyond providing logistical support, unless Mr. Trump declares martial law. The troops will not be enforcing United States immigration law — that would run afoul of the Posse Comitatus Act of 1878, unless a special exception is made.