Our country has had ample opportunity to elect a Vietnam veteran as president. It has chosen not to, time and time again. It’s not a coincidence our three boomer presidents (Bill Clinton, George W. Bush, Mr. Trump) all managed to avoid service in Vietnam. Nor is it coincidence that veterans like John McCain and John Kerry had their service records used against them on the presidential campaign trail. Even Al Gore, who spent a few months in Vietnam as an Army newspaper reporter, got closer to battle than the man who defeated him, Mr. Bush, who spent his service time stateside on Air National Guard bases.In so many ways, Mr. Trump is not a cause of diminishing respect for the military, but a symptom of it. So it is with 21st-century America and war. “Thank you for your service,” but spare the details, please.Last month, I attended a retirement ceremony at Fort Knox, Ky., for the platoon sergeant I served with in Iraq. A photograph of Mr. Trump glowering greets people at the visitors’ center there, next to the stern, cool-eyed image of (the soon departing) Secretary Mattis. We celebrated that evening with my friend’s family over dinner and drinks, and talk gradually turned to the world. I asked my friend what changes he’d noticed over his 20-plus years of military service — how different presidents have shaped his career, his multiple combat deployments.“It’s been the same for us for a long time,” he said. “Republicans in charge, Democrats in charge — hasn’t really mattered much, to be honest.” A few years ago, the author and Vietnam veteran Karl Marlantes told me in an interview, “We’re not behaving like a republic,” and his words return to me a lot when I read war news. Something is broken in our system if the machineries of war endure all strategies and policy changes, if only for the sake of enduring. There are legitimate reasons, both related to national security and humanitarian interests, to remain in Syria, as one example — our Kurdish allies pre-eminent among them.But when questions like “How long?” and “How many?” and “What’s the objective?” get swallowed up by a defense industry that essentially answers with, “We’ll handle it,” it’s no wonder that the American citizenry doesn’t engage with its military much beyond surprise homecomings at football games.