WHY NOW AND NOT EARLIER?
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REASON #1: You can't take steps like this without massive planning. For all the criticiism of US hesitancy, I don't see how such a zone would be possible until Syrians accomplish the one key thing that Libyans had done BEFORE a no-fly zone was instituted: They established in the east a substantial zone totally in their control and where the regime had been totally driven out. Now the FSA has one in Syria's north. It's far more difficult to establish a no fly zone over territory controlled by the enemy.
REASON #2: Previously fragmentation among the opposition also made such a zone impractical. The steps being taken this week in Turkey to unite the FSA into a national army also make the move easier.
A NO FLY ZONE WOULD ACCELERATE MILITARY DEFECTIONS SUBSTANTIALLY
From its contacts inside Syria's military, the FSA reports that 70 percent of the Syrian armored forces has just waiting for air cover to defect along with their equipment. I suspect that is likely. I'd also imagine the attitude among artillery troops is similar. You can't man an army when you can only draw from 10 percent of the population.
Links:
[1] //www.guardian.co.uk/world/turkey
[2] //www.guardian.co.uk/world/syria
[3] //www.guardian.co.uk/world/hillaryclinton