How long will Syria remain constrained?

Running out of time tonight, so a quick referral to a Washington Post article on the potential internationalisation of the Syrian civil wars; uprising; rebellion etc.. Add rival powers providing finance and weapons for both sides, the poisonous brew of WMDs and forays from Iranian proxies in Lebanon; we come to the oddest question of all: why jas it remained contained? What has Assad been promised or threatened to maintain such constraints at a time of national strife?