Russian aircraft carried out a bombing attack against Syrian opposition fighters on Wednesday, including at least one group trained by the C.I.A. , eliciting angry protests from American officials and plunging the complex sectarian war there into dangerous new territory. Russia ’s entry into the Syrian conflict, foreshadowed by a rapid military buildup in the past three weeks at an air base in Latakia, Syria , makes the possibility of a political settlement in Syria more difficult and creates a new risk of inadvertent incidents between American and Russian warplanes flying in the same area. And it adds a powerful but unpredictable combatant to a civil war that has already resulted in hundreds of thousands of deaths and a flood of refugees. President Vladimir V. Putin of Russia justified his country’s entry into the conflict by saying that Russia was acting “preventatively, to fight and destroy militants […]