Well, this is one hell of a bad situation for the U.S. As I understand it:
At first, a couple years ago when the conflict started, we only got involved at an incredibly superficial level when Obama made his red-line statement about direct U.S. military intervention only in the event that Assad used chemical weapons. This was strategic, and based on the thinking that Assad wouldn't use chemical weapons because he'd be shooting himself in the foot. By that, I mean that direct U.S. military involvement would be a sure-fire way to dethrone him. Assad may be kinda nuts, but he likes being in power. He wouldn't use chemical weapons, since that meant the U.S. would intervene, which would very likely result in his dethronement (and possibly worse).
Fast forward a couple years, and chemical weapons have supposedly been used (I say "supposedly" because I know that there were U.N. investigators in Syria who took samples for testing, but I don't know if they've conducted the tests and have results yet). It's unclear whether they were knowingly used by Assad, used by some of his people without his knowledge or consent, used by the rebels to try to get the U.S. involved, or some other scenario. At this point, that doesn't really matter, and I doubt we'll ever know the truth.
There are several complicating factors. First (and this is on hearsay), I've read that Russia said they would retaliate if the U.S. intervenes militarily in Syria. How and when, I don't know. Seeing as how so much of politics is verbal maneuvering in the attempt to ensure that physical maneuvering (military involvement and confrontation) is unnecessary (nobody really WANTS to go to war), should this be considered as just talk to try to discourage the U.S. from becoming involved? Even if it is just talk, I think it should also be considered as though it's not just talk, simply for preparedness' sake.
Second, none of the possible outcomes are good for the U.S. If Assad stays in power, I'm sure he won't be any kinder to the rebels than he already is. If the rebels gain power, it's not like they're going to set up a democracy just because we want them to. Apparently, the strongest rebel faction has ties to Al Qaida, so if they come into power, Syria could have--in the worst-case scenario--a radical Islamist, jihadist government that is vehemently (maybe violently so) anti-Western. That is bad not just for the U.S., but for our Western allies as well. A third possibility (which sounds incredibly unlikely and would pacify no one) is that Assad steps down, but his regime stays in power. I think a situation like that would just prolong Syria's civil war. So, as far as outcomes go, the U.S. is screwed.
A third complicating factor is that fewer and fewer of our allies are willing to fight along with the U.S. in the event of military involvement. At present, I believe only France is still willing to get involved militarily. In the event of involvement in Syria, the U.S. doesn't want to go it alone.
Factor 3.5 (which ties in to both Factor 3 and what will be Factor 4) is that Obama is (as well as so many American citizens are) hesitant to get involved militarily. The U.S. wasn't even the first country to declare its intent to intervene militarily in Syria (I believe that distinction would go to France, Turkey, and Britain). We came in a little later on, and then most of our allies dropped out. Based on the wars in Iraq and Afghanistan, some in our government's foreign policy sector have begun to realize that intervention for humanitarian reasons alone is often not worth the cost, especially considering that what replaces a tyrannical regime is not always better, and can be worse. I suppose none of this would be an issue if Obama hadn't made his statement about the red line. However, I think that's a moot point now because: 1) It's (likely) been crossed; and 2) What was he supposed to do? I have to wonder, since the U.S. is THE world power at this point in time, would a weaker condemnation and threat have been the way to go? Would it have been considered enough?
Factor 4 is Obama's decision to request approval from Congress before engaging in military action in Syria. Regardless of what he has or hasn't done mostly or solely on executive authority alone in the past, I think it's a good decision to try to do this through the proper channels.
Factor 5 is the myriad of possible consequences of getting or not getting involved. On the side of getting involved, we may have to deal with armed conflict with Russia, in addition to armed conflict with Assad's forces in Syria.
The one upside to military involvement is the precedent it would set in regard to the U.S. backing up its words with actions. If we threaten action upon Syria's crossing of the red line, and they cross it and we do nothing, it can be seen as weakness and hesitation. It creates doubt that the U.S. will act on threats it makes. This doubt may lead other nations, such as Iran or North Korea, to be willing to risk taking actions they wouldn't have taken otherwise. Taking those actions may involve the U.S. in something larger and/or worse than it may have become involved in if it had acted on its threat to Syria, thus dispelling doubts about its willingness to act on its threats. I realize this is purely hypothetical, but I don't think it's unreasonable.
The downsides to military involvement include: 1) The possibility that the removal of Assad's regime will lead to a worse government, or one that is as bad as Assad. In addition, it will likely be much more chaotic. 2) The condemnation of the U.S. by other nations and by people at home for getting involved. However, they already do this with regard to past involvement in the Middle East, so I don't think this point has much weight. Such sentiments might increase in intensity, but they won't be adding anything that's not already present. 3) As I've said before, the possibility of military conflict with Russia if the U.S. gets involved exists, although I'm unsure of how likely that actually is.
I'm sure there are other possible consequences, but I can't think of them right now, and I'm feeling mentally drained from compiling and clarifying my thoughts and opinions for the purpose of typing this post, so my motivation is starting to lessen.
I would mention backlash at home, but that will happen whether we end up getting involved in Syria or not. And frankly, so much of it is so mindlessly partisan (and mindless in general, mainly from people who don't think before they speak and don't bother trying to go beyond knee-jerk reactions) that I don't give it any weight.
Personally, I'm very much unsure of what I think would be the better option (there is no *best* option).
In summary, the situation regarding Syria is complicated and there are many, many things to consider. It's a sticky, tricky mess that currently appears to have no good outcome.
Here are links to a couple articles that, in addition to what Yatahaze posted in
>>10, have helped form and inform my thoughts on this whole mess:
http://www.stratfor.com/weekly/obamas-bluff?utm_source=freelist-f&utm_medium=email&utm_campaign=20130827&utm_term=Gweekly&utm_content=readmore&elq=c8689739f06a41be992fdfa679a51e44
http://www.stratfor.com/weekly/obamas-tightrope-walk?utm_source=freelist-f&utm_medium=email&utm_campaign=20130903&utm_term=Gweekly&utm_content=readmore&elq=def9088d16314113bbcdc8facf3971d8