Even on non-humanitarian grounds, the US and all other western democracies have a distinct interest in making sure one way or another that Assad does not win this war militarily. If Iran ultimately achieves its ambition to form a Shiite crescent consisting of Iran, Iraq, Syria and Lebanon, governed by the Iranian Ayatollahs and backed strongly by Russia and to a lesser degree by China, armed with nuclear and chemical weapons and a population driven to militancy with propaganda comparable to the works of Goebbels, we'll eventually be on the verge of the biggest global confrontation since WW2.
Sure, but I don't think that Assad really fits into that picture. He's no mystery, he's been in power for years, and his father before him. He's a Baathist, a vaguely left-wing Arab nationalist. (Not unlike Saddam, though with less megalomania.) That's how Syria originally ended up allied with the Soviet Union and with Russia today. More importantly, he's a secularist. He's actually been something of a supporter of the rights of religious minorities, since he's a mamber of a religious minority himself.
So I don't see him as a willing instrument in the formation of some Iranian-dominated Shi'ite radical religious crescent stretching west to the Mediterranean. He has formed an alliance with Iran, largely because he's cornered and needs any ally he can find. I do agree that Iran probably does hope to use him to expand its own power and influence in the region.
I acknowledge the concerns about entering into the Syrian war, the risks of playing into Al Qaeda's hands
Or certainly Islamist hands, if not al Qaeda's. When rebels take over villages and towns in Syria, they often establish Sunni religious courts and enforce Shariah. There have been many reports of ethnic cleansing, where Christians, Druze and Alawites are forced from their homes or killed. (To be fair, there are also reports of Sunnis being 'cleansed' from towns in Assad's increasingly isolated Alawite enclave along the coast. Both sides are doing it.) There are reports of rebels setting up roadblocks, stopping cars and buses, with non-Sunnis separated out, put up against a wall and shot.
Things are getting very ugly in Syria, where angry and violent polarization has reached the point where all the dreamy talk about "negotiations" and a "political solution" may no longer be realistic. Each side feels aggrieved, and each side wants to exact bloody revenge. This isn't the stereotypical "Arab Spring", with hip and wonderful internet-using students and liberals demanding freedom and Western-style democracy. It's a corrupt and brutal family dictatorship that's made lots of enemies over the years, fighting a disorganized collection of medieval-style religious fanatics. The fighting is growing more and more savage by the day.
And while I don't really see Assad willingly copying the Tehran mullahs (his long-established secular style and rather-nominal personal religiosity seem too different), I can easily see the rebels trying to install something that will make Egypt's Muslim Brotherhood look downright progressive. To be honest, they worry me more than Assad does.
The idea that we can somehow support the 'good' rebels seems kind of doubtful to me. That will only suck us into a three-way civil war, where our 'good' rebels fight both the more numerous bad rebels and either Assad, or if we get our way and he's overthrown, Alawite and other religious minority militias, struggling to the death to defend their own communities. These sides are likely to be disunited and fighting among themselves as well. In other words, the two most likely outcomes might conceivably be either: a) Syria devolving into a Somalia-style failed state consumed by a post-apocalyptic battle of all-against-all, or b) a de-facto partition, with Assad holding out in part of the country (most likely the west along the Mediterranean, around Hama and Homs, and south around Damascus), and the rebels in the north, around Aleppo and east along the Euphrates. (The Kurds will probably carve out a statelet in the far northeast as well.) If that happens, we might see large-scale exchanges of population as ethnic cleansing expands.
