From Lucas Tomlinson:

It is the first time Russia has deployed the SA-23 system outside its borders, according to one Western official citing a recent intelligence assessment. The missiles and associated components are still in their crates and are not yet operational, according to the officials…

The SA-23 can fire two different types of missiles. A smaller missile is used against aircraft and cruise missiles and is known by NATO as Gladiator. The larger missile is used against intermediate-range ballistic missiles and jamming aircraft and is known as Giant. Both missiles use the same type of warhead containing over 300 pounds of explosives, according to military-today.com.

Three years ago, President Obama weighed military action against the Assad regime’s chemical weapons facilities as well as airbases housing the regime’s attack helicopters and jets. US Navy ships in the eastern Mediterranean were prepared to launch Tomahawk cruise missiles in a limited strike to cripple the regime.

Russia deployed a separate air defense system, the S-400, to Syria after a Russian jet was shot down by a Turkish warplane last November.  Since the S-400 deployment, the U.S. military has been careful about flying  manned aircraft inside the range of the system, despite repeated pledges by the US military that its airstrikes in Syria are focused on ISIS, not the Assad regime.