The Syrian Civil War could go on a long time:
[Most] Alawites and possibly other minorities, view the civil war as threatening the future existence of their communities. It is also viewed as beyond compromise by many rebels. Assad’s regime rules by fear, and that kind of system requires people to be punished harshly for disloyalty and to keep others in line. Forgiveness is viewed as weakness, and brutality is the default approach of the Syrian regime for all problems. Rebels who lay down their arms face death, and they know it. This leaves the international community with two sides which have no reason to compromise and every reason to continue fighting. Neither side will compromise in the middle of a conflict that they view as potentially detrimental for themselves, their families, and their communities. Moreover, the death of any leader, including Assad, will probably not result in a more compromising successor under such conditions. To make matters worse, the collapse of the Assad regime would probably only lead to a new phase of the civil war in which very different kinds of rebel groups fight each other for power.
Unfortunately, at the present time, diplomatic solutions also seem unlikely, and the war could well go on until all sides are too exhausted to continue. The Lebanese Civil War lasted 15 years under similar circumstances.
Can Assad really carry on the fight for that long? The author notes that Assad's forces are using indiscriminate firepower to preserve loyalist lives:
Since the regime does not want to use up its Alawite soldiers and militiamen in battle, these troops often use firepower, including artillery and airpower, to strike at rebel forces despite the tremendous collateral damage inflicted by the near indiscriminate use of these weapons.
But if this strategy is preserving their lives, one shudders to imagine the toll with a strategy that isn't trying to preserve Alawite lives. In September 2013, the government casualties were just astounding:
[It] is believed that 27,000 soldiers and 17,000 pro-Assad militia have also died, along with over 21,000 rebel fighters.
Remember, the fighting as really only gotten going in the last two years after protests and repression escalated to civil war. So Assad's forces have endured 44,000 dead in about a year and a half of significant fighting.
Indeed, as I wait to post this, new statistics put the total at over 130,000 dead, with government force casualties standing at 52,290, including over 500 from Hezbollah and Iran's Shia foreign legion. That's 8,000 more in about 3 months. And this is a rate consistent with trying to avoid military casualties?
I know, Assad has been on offense since the spring 2013. But he is fighting for his corner of Syria and has enjoyed an influx of militias and foreigners to spearhead attacks.
Barring a rebel collapse--and is that likely given their fear of retribution if they lose?--I don't think Assad has the numbers to really pacify his corner let alone reconquer all of Syria.
Does the war in Syria have 10 or more years left? That's quite likely. The war in Iraq is still going. As is Afghanistan.
See also Colombia and the Philippines for that issue, not to mention Sudan. This list could go on.
People speak of the Assad loyalists fighting hard out of fear of losing. That same force will keep rebels fighting, too, remember.
But I think it is too hopeful to think that all sides will fight to an exhausted stalemate. One side will break. At some point, even fear of retribution won't keep people fighting.
I just don't think Assad's small population base can endure ten years of this kind of blood letting. And I don't think Iran's Shia foreign legion and Hezbollah can compensate for this weakness. I could be wrong, but I think Assad's side is more likely to break than the rebel side--if the rebels receive enough outside support.
Nor do I think our options are limited to coping with possible spillovers around the region. If the war is likely to end with an Assad collapse and subsequent factional war, isn't it better to get past that Assad collapse faster?
Indeed, trying to accelerate the defeat of Assad could be the best way to cope with the spillover threats to neighbors.
And remember what Lebanon looks like, if that seems like our best case after an exhausted Syria simply stops fighting in place. Do we really want Syria to join Lebanon as a fractured state with factions carrying on their own foreign policies as effective states within the formal state?
No. Given that this war will likely go on for a long time, we should make efforts to support those we'd prefer to emerge victorious, even if their chances look bleak now. In a long war, their current status isn't written in stone, is it?
Having a clear victor will also be superior on a humanitarian level in the long run, remember. I know there is a strong strain in our society that thinks we make things worse if we intervene, but Syria's civil war without us more than nominally involved is way worse in a way shorter amount of time than Iraq ever got with our direct intervention (and recent problems are more from our lack of involvement in Iraq after 2011 and our failure to act in Syria).
This doesn't mean I support direct American intervention. Even if I thought we should do it, I wouldn't advise it because I don't think we have the national morale to intervene directly for the time required to win. But it does mean preparing for the long haul and sticking to it when the going gets rough.