Michael Brendan Dougherty sums up  why Trump can’t be trusted to conduct foreign policy:
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.
I’ve said before that Trump tends to be all over the map on foreign policy. He can be all over the map because he is shameless, he doesn’t know much about it, he isn’t that interested in it, and he isn’t bound by anything resembling a coherent foreign policy vision. If you want a break from the Washington foreign policy consensus, it is possible to find statements from Trump that suggest that he agrees with you some of the time, and it is also very likely that he has said just the opposite in the same week, hour, or sentence. His die-hard opponents and supporters both want to believe the statements Trump makes that fit their view of him, and many would rather ignore or explain away the statements that don’t fit.
If he says he is for a “safe zone” in Syria, it is a safe bet that he doesn’t understand what that involves, but that doesn’t mean he wouldn’t support one once someone told him what it was. He objects to having an adversarial relationship with Russia, but has no interest in making any of the accommodations that having a cooperative relationship would require. In one breath, he’ll worry about nuclear proliferation, and in the next suggest that he is fine with more states acquiring nuclear weapons. When it matters, he has no problem with the latest unnecessary war, and only after it is too late does he discover that he is against them. Weirdly, he is a fan of making deals in the abstract, but hates every deal that every president has ever made.
The only consistent views I have found across all of Trump’s various positions is that he always thinks America is getting a raw deal regardless of the details and he always wants to plunder other countries when given the opportunity. Needless to say, this isn’t someone who is interested in restraint as a virtue or as a grand strategy, and no one should pretend otherwise.