It is time for a serious assessment of what a Donald Trump presidency would mean for the United Nations. For most of this year, this prospect has seemed little more than a topic for passing drollery. In May, for instance, I wrote that “as president, Trump will love the U.N.: He loves bloviating, so he should feel right at home in Turtle Bay.”

With the U.S. elections just over 40 days away, this seems less amusing. Democratic nominee Hillary Clinton is still the frontrunner, but opinion polls suggest her lead is narrow. U.S. officials and diplomats in foreign capitals are scrabbling to prepare for the possibility of a Trump victory. Their counterparts at the U.N. are distracted by the contest to replace Ban Ki-moon as secretary-general, to say nothing of recent spikes of violence in Syria and the Democratic Republic of Congo. But they should be doing their own contingency planning for Trump, particularly with regard to what world powers could do to prop up the U.N. if he attempts to take it apart. ...