golJERUSALEM  – Israel shot down a Syrian warplane on Tuesday, saying the aircraft crossed the battle lines of Syria’s civil war and flew over the Israeli-held Golan Heights, perhaps by accident. The incident coincided with but did not appear to be directly related to air strikes the United States and Gulf Arab allies mounted on Islamic State strongholds in Syria. But it presented another challenge to Israel’s oft-stated desire to stay on the sidelines of a conflict on its northern doorstep, in which al Qaeda-linked Nusra Front rebels took over a border crossing on the Golan last month. The Israeli military said its U.S.-made Patriot missile air defence system shot down a Syrian Russian-built Sukhoi fighter plane that had “infiltrated Israeli airspace” over the territory, captured by Israel in the 1967 Middle East war. More