Donald Trump’s second state visit in six years starts today at King Charles’s Windsor estate in style: a carriage ride (past Harry and Meghan’s old haunt of Frogmore Cottage, as it happens), a lavish evening banquet and panoply of military bands and fly pasts. It is a massive ceremonial courtesy to a mighty US President by an embattled ally, on behalf of a Prime Minister who needs a hefty dose of political vitamins. For all the festivity, there is not much sign of significant movement on tariffs on the “big enchiladas” – such as relief on the punitive tariffs now...