Crown Prince Mohammed bin Salman prides himself on strong relations with President Trump, but Mr. Trump’s second term is posing serious dilemmas for the young Saudi leader, known as MBS. Mr. Trump wants low oil prices. Saudi Arabia needs high prices to fund its plan to wean the kingdom off oil. Mr. Trump has resumed his maximum-pressure campaign against Iran, undermining the crown prince’s efforts to reduce tensions with Tehran that could lead the clerical regime to lash out at Saudi Arabia. Most disruptive is Mr. Trump’s proposal to turn Gaza into the “Riviera of the Middle East" by removing...