Mr. Hariri, until he announced his resignation on Saturday, had shown no signs of planning to do so. Hours later, on Saturday evening, a missile fired from Yemen came close to Riyadh before being shot down by Saudi Arabia. It then emerged that the week before, Jared Kushner, had visited Riyadh on a previously undisclosed trip and met until the early morning hours with the crown prince.

It was not immediately clear why the five countries decided to take this action now. Last month, Qatar’s state news media published comments attributed to the emir, Sheikh Tamim bin Hamad al-Thani, referring to  tension with Washington over Iran policy and saying Mr. Trump might not  be in power for long. Qatar denied the comments, saying it had been the victim of a “cybercrime.” But most analysts pointed to President Trump’s recent visit to Saudi Arabia.

The diplomatic situation had been looking bright for President Bashar al-Assad of Syria. With the help of Russia, he had consolidated his power, the rebels were on their heels and the United States had just declared that ousting him was not a priority. Dr. Monzer Khalil, the health director for Idlib Province, said such extreme tactics are designed to demonstrate the government’s impunity and to demoralize its opponents.

The United States blamed the Syrian government and its patrons, Russia and Iran, on Tuesday for one of the deadliest chemical weapons attacks in years in Syria, one that killed dozens of people in Idlib Province,  including children, and sickened scores more.