Diplomacy is often about appearing to be doing something until the time is ripe for a deal to be done.

Now, with Mr. Assad’s forces having suffered a string of losses on the battlefield and the United States reaching at least a partial rapprochement with Mr. Assad’s main backer, Iran, Mr. de Mistura is changing course. Starting Monday, he is set to hold a series of closed talks in Geneva with the warring sides and their main supporters. Iran will be among them.