With President Bashar al-Assad of Syria facing battlefield setbacks, diplomats from Russia , the United States and several Middle Eastern powers are engaged in a burst of diplomatic activity, trying to head off a deeper collapse of the country that could further strengthen the militant group Islamic State. Russia, Mr. Assad’s most powerful backer, has built new ties with Saudi Arabia, a fervent opponent, and even brokered a meeting between high-ranking Saudi and Syrian intelligence officials. On Tuesday, the Saudi foreign minister, Adel al-Jubeir, met with the Russian foreign minister, Sergey V. Lavrov, in Moscow, wrangling over the fate of Mr. Assad. Unusual meetings have come in quick succession. Last week, the top Russian, American and Saudi envoys held their first three-way meeting […]