LONDON — President Trump sat down in a gilded chair beside President Emmanuel Macron of France on Tuesday, prepared for what has become a ritual of sorts on his home turf at the White House: He holds forth as another leader is left to smile stoically through his jokes, jabs and insults. But Mr. Macron changed the script. By the time their 45-minute appearance at the American ambassador’s residence in London was over, the French leader had managed a rare role reversal, putting Mr. Trump on the defensive about his vision for NATO and his handling of a military conflict...