When Turkey downed a Russian Su-24 warplane on Nov. 24, it did not just sour relations between Ankara and Moscow: It gave Russia a reason to build up its air defense capabilities in Syria. Before the incident, the Russian air cover burden depended on four deployed multirole Su-30 aircraft, which escorted Russian strike packages. A week after the downing, Russian Su-34 fighter-bombers in Syria were spotted carrying R-27 air-to-air missiles. Russia also rushed advanced surface-to-air missile systems to Syria. Russia has enhanced these air defenses to prevent other countries from forcefully entering the Syrian conflict. ...