Already, the UN estimates that over 100,000 Syrians have died in the fighting (all sides, including troops and rebel fighters in addition to civilians). While Assad is hanging on after reclaiming the initiative a few months ago, the rebellion doesn't look like it is in danger of defeat.
The rebels are already twice as numerous as the insurgents and terrorists were in Iraq:
[Many] (over twenty percent) of the 50,000 rebel gunmen are Islamic radicals[.]
Al Qaeda is a major player in the jihadi contingent.
Given that opponents of the Iraq War charged that our invasion of Iraq caused al Qaeda to go to Iraq, this Syria development should be a reminder that al Qaeda goes where it can and not where we are. It is perhaps a comforting believe in a large part of the population that defeating jihadis is simply a matter of staying away from regions the jihadis claim should be there territory, but it just isn't so, is it?
This time, our failure to pounce on Assad's difficulties to support rebels has given the jihadis time to move into Syria. In a bit of cosmic justice, I imagine the networks of Islamists that existed to funnel jihadis through Syria into Iraq has had a role in getting a lot of jihadis into Syria to fight Assad. But that doesn't mean the jihadis are less of a problem for us.
At some point, whether after Assad wins the war and regains control of Syria (low odds, and it would take a lot of time to build up the army needed to reassert control), overthrow of Assad (less unlikely, but with continued Iranian and Russian support, Assad seems able to hang on to some part of Syria), or fragmentation--either formal or informal--we will have to deal with the jihadis who survive the war.
Failure to fight these jihadis could threaten Iraq if jihadis turn around to reinforce their brethren already surging violence against Iraqis--they are strong enough to stage a jail assault that freed 500 jihadis.
Failure to fight these jihadis could threaten Turkey if jihadis infect the Kurds who are coming to terms with the Turkish government.
Failure to fight these jihadis could threaten Lebanon if the jihadis chase Hezbollah to continue the fight inside Lebanon.
Failure to fight these jihadis could threaten Israel if the jihadis use their position in Syria to strike Israel or move into Gaza or southern Lebanon and spark a war with Israel by being holier than other jihadis.
Failure to fight these jihadis could threaten Jordan if the jihadis infiltrate that state and exploit ethnic divisions and poverty.
Faioure to fight these jihadis could threaten us if the jihadis organize even a portion of Syria into a sanctuary where they can set their sights on American targets.
We didn't intervene a couple years ago when our power might have been decisive before jihadis entered the fight. Now the fight in Syria is part of the war on terror whether we like it or not. We need to organize forces to fight the jihadis inside Syria. Ideally, we strengthen the non-jihadi elements in Syria to fight the jihadis to keep the problem from being immediately exported abroad.
The tide of war is not receding.