Sadiq Khan, London's recently elected mayor, is eager to build trans-Atlantic urban partnerships. An energetic fan of great cities around the world, Khan has expressed admiration for such stateside peers as New York's Bill de Blasio and Houston's Annise Parker. He's also impressed his American counterparts. The U.S. Conference of Mayors is so enthusiastic about the 45-year-old former minister of transport that the group invited him to give a keynote address at its annual conference this summer.

In a normal year, the winner of the London mayor's race would have gone unnoticed by the vast majority of Americans. But Khan, who ran under the Labour Party banner, has just become the first Muslim mayor of London, and he has also plunged headlong into a war of words with Donald Trump, deftly setting up the presumptive Republican nominee for his first major international embarrassment.