= new reply since forum marked as read
Russia has sent 12 SU-24 attack aircraft, 12 SU-25 ground attack aircraft and four Flanker fighter jets.
The Russians have sent to Syria FOUR of their top notch fighters, the SU-27 Flanker. The Flanker is considered equal to the US F-15 in overall maneuverability and basic flying characteristics but has inferior electronics. Thus if an attack by F-15s from the US Air Force or Israel (or attacks by F-18 from the US Navy), the SU-27 is the plane to counter such attacks. In such situation who sees who first will determine the fight, thus US fighters given superior Radar and Electronics have an edge, but with the SU-27 that edge is the smallest it can be today.
https://en.wikipedia.org/wiki/Sukhoi_Su-27
Sending SU-25 also makes sense, it is the Russia equivalent of the American A-10, a very good attack plane, thus in many ways the perfect plane to use against ground forces:
https://en.wikipedia.org/wiki/Sukhoi_Su-25
The big question mark is why the SU-24? The SU-24 is a swept wing bomber designed in the 1960s. It is considered a slightly smaller F-111 type plane (i.e, a bomber with some fighter capacity) It was a feared plane in the 1970s and 1980s due to its range, its bombing capacity and it electronic kit (It was one of the few Soviet Planes of the 1970s that was considered as accurate as US attack planes). It is a little long in the tooth today, but it is more a long range interdiction plane, i.e a plane to bomb well behind enemy lines. It has a combat radius of 615 KM carrying up to 8000 KG of weapons. It was designed to get to its target flying at low attitudes to avoid radar and hit its target on the first pass and then go home. The only reason I can think of, is someone wants the ability to hit deep into someone's else's territory OR hit a Naval Target with a long range missile.
https://en.wikipedia.org/wiki/Sukhoi_Su-24
Given ISIS capacity the SU-24 makes perfect sense, it is a attack plane design to hit targets picked out by ground forces as they appear and to hang around till they do appear. The F-27 makes sense, if Syria fears fighters from the US, Turkey or Israel (or all three, through we can NOT throw out Saudi Arabia and its F-15s).
Su-24s are the question marks in my opinion. It is NOT a weapon to use against a mobile enemy but against supply points, headquarters and stationary targets like fixed artillery or air bases, bridges, dams and other infrastructure needed to keep up a supply chain.
What is Syria afraid of? The SU-24 is not a weapon one would request if you have an enemy like ISIS, something else is up, what it is I do not know (and it may just be paranoia by Assad, i.e. he wants the ability to hit Israel or maybe Saudi Arabia).
to Syria from Russia.
More like, imo, Moscow is dictating to Assad what it wants in a military build up as a trade off for assisting Assad's attempt to stay in power.
Two superpowers rubbing shoulders with each other in the middle of a 3-way civil war. What could go wrong? 
