The most striking thing about President Trump’s proposal for a “Board of Peace,” a new group he has billed as a global conflict-solving body, is not its billion-dollar permanent membership fee or the eccentric list of nations, such as El Salvador, Belarus and Saudia Arabia, that have apparently signed on. It’s that for the first time, the United States — the primary architect of the United Nations — is openly experimenting with a rival body at least nominally aimed at peacemaking. “I’m a big fan of the U.N.’s potential,” Mr. Trump said last week, “but it has never lived up...