In his last phone call home, Lance Cpl. Gregory Buckley Jr. told his father what was troubling him: From his bunk in southern Afghanistan, he could hear Afghan police officers sexually abusing boys they had brought to the base. “At night we can hear them screaming, but we’re not allowed to do anything about it,” the Marine’s father, Gregory Buckley Sr., recalled his son telling him before he was shot to death at the base in 2012. He urged his son to tell his superiors. “My son said that his officers told him to look the other way because it’s...