Soon after Portugal’s António Guterres was picked to succeed Ban Ki-moon as the United Nations Secretary-General, commentary on the major challenges awaiting him was dominated by the Security Council’s paralysis in resolving the Syrian conflict. There was also mention of climate change, North Korea’s nukes, Africa’s wars and the refugee crisis engulfing Europe. Nobody had anticipated then that Mr. Guterres would be presented with a new challenge of a different kind: Donald Trump. Last week, reacting to the decision by the Obama administration not to veto the resolution condemning Israeli settlements in occupied Palestinian territory, a resolution passed 14-0 in the 15-member Security Council with American abstention, a miffed Mr. Trump trashed the UN as “just a club for people to get together, talk and have a good time.” A not-so-veiled threat followed: “As to the U.N., things will be different after Jan. 20th.” That is when Mr. Trump officially succeeds President Obama as U.S. Pres...