The best reason for conservatives to withhold their support from Donald Trump is that he cannot be trusted to lead America's foreign policy or command its military. For many this is so self-evident given Trump's character and the lack of normal political constraints under which he operates, that there's no need to elaborate. But some need convincing.
Many of Trump's supporters are happy that he seems to have taken on the foreign policy orthodoxies of his party. They want an America that doesn't waste trillions of dollars in fruitless efforts like turning Iraq into a democracy, or helping Libyan rebels only to see that country become an operating base for ISIS. They know that Hillary Clinton's instinct is to use American air power in the name of human rights even if it leads to pro-Islamist outcomes, whether in Kosovo or Libya. In fact, I want and believe the same things.
But there aren't strong reasons to believe Trump is any better than Clinton when it comes to making peace. In fact, he may be much worse.
Trump supported all the dumb wars and interventions that he now claims to have been against. He supported President George W. Bush on invading Iraq. Though he says he was against it, Trump supported the intervention in Libya in the most anti-realist terms possible when he said, "We've got to go in and save these lives." He is just all over the place, saying that we shouldn't be involved in Syria, and then a few minutes later saying that the U.S. should create safe zones in Syria.
The simple explanation for these changes is that Donald Trump hasn't ever thought hard about foreign policy; he simply has an instinct for where public opinion is at any moment on any given war and runs ahead of it. That's fine for someone holding forth at the bar stool. It's disastrous in a man who sits in the Oval Office.
Almost the entirety of the foreign policy establishment is against Donald Trump. That includes not just the hawkish neoconservatives, but also the foreign policy realists who would be the only group of advisors that could shape Trump's "America First" foreign policy into a real alternative to the last 25 years of post-Cold War interventionism. He would simply be disarmed of the kind of expertise needed to run America's foreign policy. Getting his way with the full-time employed members of the State and Defense Departments will prove difficult and lead to upheaval or administrative gridlock, at best.
Trump has named a handful of under-qualified foreign policy hands. Some of them are quite alarming in themselves, like Walid Phares, who has repeatedly sounded the bell that Muslims have a secret plot to take over America and impose sharia law.
Trump seems to believe any and every conspiracy theory that passes by his nose — not just that vaccines cause Down syndrome or that Barack Obama may be a secret Kenyan. He has said he believed that Obama struck a deal with the Saudis to keep oil prices low ahead of his re-election in 2012. If you thought that it was bad when the Bush administration came to believe its own bad intelligence, imagine what a Trump administration would do when the president wants to believe something. Beyond that, Trump has promised that American military members will commit war crimes and other acts of torture on his say-so, merely because he is Donald Trump.
America is already too quick to use its military power to try to shape outcomes in far-off places throughout the world. This defect would only be exacerbated if a person with Trump's twitchy sense of honor and aggression steps into the role of commander-in-chief.
The very fact that most of the elected officials of the Republican Party — including those that once called Trump a "cancer," a "con artist," or an "erratic individual" who can't be trusted with America's nuclear arsenal — have lined up to endorse him or even become his vice president shows that our political class is unlikely to resist him doing something truly dangerous if he is perceived as popular. Too many, when faced with the choice between their high principles and Trump, chose Trump as an expediency. We should not tempt them with a choice between their president and the security of our nation.