FG

At present two major fears inhibit the West from arming the Syrian people so they can defend themselve fully:

1.  The fear that Islamists will get their hands on western weapons and eventually turn them on the West.

2.  The fear that Iran would intervene.  

Regarding the first, I think it more than likely that--once Assad falls--such weapons will be used against regimes that enabled the slaughter (Russia and Iran) probably targeting their their proxies first (Lebanon's Hezbollah and Iraq's Al-Maliki).  Is that such a bad thing for the West?  We'd also reduce the chances that Islamists could exploit our present non-action for political gain later.

Regarding direct IRI intervention, I want to argue that if arming the rebels provokes it the probable consequnces to the West would be more desirable than harmful. 

POTENTIAL CONSEQUENCES FOR ASSAD:Like Mussolini after his rescue by German paratroops, Assad will look a puppet kept in power solely by the presence of any ground forces from Iran.   Among Assad's own troops--as among Italians in WWII--defections will increase immensely when--not if--the chant of "Khamenei's imperialism" spreads.

POTENTIAL CONSEQUENCES FOR "CZAR" KHAMENEI: A deeply unpopular and costly intervention would almost certainly do for a despised mullocracy what World War "accomplished" in 1917 for the Czar and for Russia's Social Democrats.  

The Islamic Republic of Iran woud have entered a war under conditions in which it cannot afford, economically or military, to get bogged down. In that sense,  its state of readiness is a poor as that facing Czar Nicholas II in 1914.  Yet that is exactly what is likely to occur, thereby increasing hardships at home, draining an already depleted treasury and demoralizing the troops.  Many will arrive already disenchanted.  Those feelings will grow along with the mass casualties that urban warfare inflicts (western arms for the FSA can see to that).

Why die to just maintain two tyrants in power?  As conditions worsen, the mass of troops will  be happy with any excuse to leave.  If rebellion breaks out at home Khamenei will miss their presence.  If he recalls them, will they be any more reliable than Czarist troops who abandoned the eastern front in 1917?

CONCERNING ANOTHER DETERRENT: SYRIA'S SOPHISTICATED AIR DEFENSE

This is a legitimate western concern when it comes to avoiding Libya-style invention so avoid the latter.  Losses would be substantially higher.  Geography and population density also makes substantial collateral damage (i.e., civilian casualties) very difficult to avoid no matter how precise one's weapons.  

Setting up a PARTIAL no-fly zone north of Aleppo would be substantially less difficult.  For areas further south, a combination of unmanned drones working in coordintion with well-armed FSA ground troops could do wonders at a place like Aleppo's airport.  Such drones would also eliminate a rationale which deters many Syrian armored troops from defecting along with their vehicles at present.  To stay is to die.