Moscow has said it will deliver several batteries of Russia's advanced S-300PMU air-defense missile system, purported to be one of the world's most powerful anti-aircraft weapons.
The Americans have repeatedly said that Syria has one of the most formidable air-defense networks in existence, largely consisting of Russian missile systems delivered over the last few years.
But since late January, Israel's air force has carried out three strikes against Russian and Iranian weapons cached in Syria -- and supposedly destined for Assad ally Hezbollah in Lebanon -- and wasn't challenged by Syrian air defenses once.
That was largely because the Israelis fired stand-off precision-guided munitions at their targets from outside Syrian air space from the skies over neighboring Lebanon. That violated Lebanese airspace -- something the Israelis often do -- but not Syrian airspace.
Whether the Syrians will activate air defenses the next time Israeli warplanes attack remains to be seen. Nor is it known whether they would open fire on U.S. or allied aircraft enforcing a no-fly zone aimed at cutting off aerial resupply for President Bashar Assad's forces and allies from Russia or Iran.
Even so, a no-fly zone, enforced by the U.S. Navy and U.S. Air Force using stand-off weapons to avoid direct contact with Syrian forces, would have a critical effect on Assad's ability to continue the war against those battling to end his family's 43-year rule.
But, possibly more importantly from the U.S. point of view, it would also be a major blow against Iran and its ability to support its strategic Arab ally and project power in the Levant and threaten Israel.
The allied no-fly zone imposed against Saddam Hussein after the 1990-91 Gulf War was pretty much one-sided with U.S., British and French aircraft rarely challenged.
The no-fly zone imposed on Libya in 2011 by the United States and its NATO allies, was of much shorter duration, but played a major role in the rebel victory over Moammar Gadhafi by knocking out his air defenses.
Syria's are considered far more sophisticated and effective.
Much of Syria's air defenses could be devastated by naval and air missile bombardment, but military analysts say they could still provide a stiff challenge.
Assad has around 300 flyable combat jets, mostly outdated MiG-21s, 23s and 25s with 35 more advanced MiG-29s interceptors, though these are heavily outclassed by U.S. frontline aircraft.
But its missile defenses, which the Russians have been upgrading since an Israeli air raid electronically zapped Syrian air defenses and knocked out a Syrian nuclear reactor Sept. 6, 2007, are arguably more effective.
The arrival of S-300s, which can track 100 targets simultaneously and engage 12 at a time, would clearly pose a serious threat to U.S. or allied aircraft. However, it would take a sizeable airlift to carry the five or six batteries, and possibly 100 missiles, to Syria and probably several weeks to deploy them with trained Syrian crews.
So if Obama does opt for decisive intervention, after weeks of inaction, and impose a no-fly zone, obviously the sooner he does so the better.
"The air line of communications between Iran and Syria is a key vulnerability for Iranian strategy in Syria," observed a May report by the Institute for Study of War.
"Iran would not be able maintain its current level of support to Assad if the air route were interdicted through a no-fly zone or rebel capture of Syrian airfields."
Tehran, and possibly Russia as well if it were so inclined, could conceivably use a ground route via Iraq, Syria's eastern neighbor, but these could be cut by Syrian Sunni rebels and their al Qaida allies in Iraq.
Resupply by sea through Syria's main ports of Latakia and Tartus, where the Russians have a naval base, is problematic since sea lanes can also be blocked.
"Given the limitations of sea lines of communication ... air shipments remain Iran's most feasible method of supplying the Assad regime," the ISW analysis noted.
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