Edward Luce: “Donald Trump had claimed to amend a U.S.-Saudi nuclear deal on his Truth Social site the day after it was signed. That the deal was controversial is one thing. That he thought he could change it without asking the Saudis was another. Trump keeps making fantasy announcements. His grip on power is suffering as a result.”“That might seem paradoxical for a U.S. president who has done more than any other to create an all-powerful presidency. But power is the ability to shape the world to your advantage. This should not be confused with constantly declaring how powerful you are.”