Russia has delivered the first S-300 surface-to-air missile system to Syria on October 2nd. According to Russian sources, the unit will be controlled by Russian advisors, a precaution Moscow took to prevent a misfire that downed the Ilyushin20 Electronic Surveillance aircraft last month.
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Having Russian officers on site would limit the Syrian freedom of action, but also put Russians in danger, if offensive countermeasures are taken against those sites, as Israel did against Syrian SA-17, and mobile SA-22 Pansir sites that challenged Israeli warplanes on previous attacks.