there is a war of world between Russia and Israel – usually quite a pacific relationship out of respect.
After the recent US and Israel attacks, Russia is negotiating to send to Syria its more advanced S300 anti aircraft missile. Not as advanced as the S400- S500, but a 10 year technological jump in respect of what Syria has available now.
The issue stands that Israeli (and other) aircraft are currently shooting at Syria from the airspace of the neutral Lebanon (technically, an invasion per se).
The current Syrian systems do not have the range to shoot the aircraft, they can jut try to intercept the missiles.
The S300 has a range of circa 250km, so it can hit airplanes above Lebanon and Israel – so it is a game changer.
Also the S300 can be operated only by the Russian Army due to expertise – so an Israel attack on the systems before operational would create Russian Army casualties.
While President Putin can accept Russian mercenaries casualties, he cannot accept Red Army casualties.
Hopefully it is only a negotiating tactic to stop the lost Western war against Assad.
investorpolitico 