Chaos in Syria? Call it the last of the big Middle East surprises — after all, before jihadist rebels pounced on Aleppo and overran Syria’s largest city, the Syrian dictator and president, Bashar al-Assad, controlled only 60 percent of the country. What happens in the coming weeks could impact the Assad clan’s half-century grip on power but is potentially more consequential for the regimes that run Russia and Iran.