The war in Iran is threatening a fragile détente between China and the United States, with the two powers now moving to postpone a much anticipated summit meeting after President Trump demanded that China send warships to the Gulf. Mr. Trump on Monday said that he had requested that his visit to Beijing at the end of the month be postponed because of the war. Just a day earlier, he threatened to delay the meeting if China did not contribute warships to end Iran’s de facto blockade of the Strait…