If there was ever a time for external US-led intervention in Syria that has passed; IIRC we discussed this two years ago, mainly in the context of imposing no-fly zones and internal safe havens.

J Wolfsberger posted just:
I'd suggest that the most we should or effectively can do is help provide humanitarian aide and security for safe zones where those who wish can escape the slaughter.
Yes we, the West, could help with humanitarian aid, although I do wonder why the rich Arab nations have not been able to sign all the cheques.

'Security for safe zones' is far more problematic. Safe zones outside Syria maybe easier, although both Jordan and Turkey have considerable numbers of refugees. Lebanon has fewer. Will 'security for safe zones' mean preventing their use as rear bases by the insurgents rather than guarding them against regime coercion?

Personally I don't think the West should undertake such a 'security' role outside or inside Syria. A UN 'blue beret' presence I expect would be opposed by Russia and China; assuming Jordan and Turkey sought that.

The Western experience in the ill-fated MNF in the Lebanon, even before the attacks on French paras & US Marine bases, is a more likely template. Oddly at least one UK analyst ignores that:http://www.bbc.co.uk/news/world-middle-east-22967636