Four rulers of a six-member alliance of Persian Gulf states will likely be absent from a summit hosted by President Obama this week at Camp David. In a statement Sunday, Saudi Foreign Minister Adel al-Jubeir said Thursday's summit coincides with a humanitarian cease-fire in Yemen, where a Saudi-led coalition is fighting Shiite rebels known as Houthis. He said Crown Prince Mohammed bin Nayef, who is also interior minister, would lead the Saudi delegation and the king's son, Deputy Crown Prince Mohammed bin Salman, who is defense minister, will also attend. Obama had planned to meet Salman one-on-one a day before...