The Trump court is a royal progress that moves between Palm Beach and the White House, for the most part in private planes. But the interests of the US government require that at least some of its members be willing to travel farther afield than Florida. Trump talks of putting the US economy behind a great tariff wall, but he also wants deals, which means he needs dealers. America’s official chief diplomat is the secretary of state, Marco Rubio, but so far his activities have been fairly limited. Instead, the role of principal US emissary is currently filled by the unlikely figure of the property developer Steve Witkoff.