There are no good courses of action the US can take in Syria. No matter what we do, it will be wrong.

In a surprise that would have been unthinkable just a few years ago, it turns out that Assad, the vicious dictator of Syria, is our natural ally, at least more so than the (primarily) al Qaeda insurgents trying to overthrow him.

And yet it makes no sense to support him since he is not only the ally of Iran, but succoring him also provides the Russians, not the US with biggest benefit by securing their Middle East interests and the only naval seaport they have in the region.

Providing humanitarian aid is a good, albeit weak strategy and make the US look weak. Meanwhile, insurgents who are of a moderate bent are being decimated in a civil war triangle by the increasingly powerful al Qaeda insurgents who most benefit by US aid.

But now we have unfortunate "red line" of alleged chemical weapons use by the Syrian government. If it proves true the US cannot ignore it and must take swift and decisive punitive action. What are the options?

1) Step up arming and training the rebels with good weapons including shoulder-fired munitions for taking down aircraft. This option has the advantage of being cheap and easy to implement, but it will be seen as impotent and will take time to have any effect and Assad may win before it pays dividend.
Cost: a few billion dollars
Time to success: months or even years, one year minimum, victory in doubt
Casualties: US, none: Syrians, in the thousands or tens of thousands
Estimated success probability: 40 - 60, currently Assad has the upper hand, and their is no guarantee the US support will not be used by the insurgent factions against each other or be timely enough to win the war

2) Bomb various targets with missiles from a distance to punish Assad, degrade his infrastructure but not overthrow the regime. While more costly than aid, and decisive, this mechanism is still cheaper than invasion and limits US involvement, but it sure to anger the Russians and Chinese who may actually step up their support of the Assad regime

Cost: probably 20 billion dollars
Time to success: Depending on the intensity or frequency, the goal may be achievable quickly if the goal is simply to punish Assad. More far reaching goals such as regime change would not be guaranteed and take longer, more bombings
Casualties: US, a few from downed aircraft, mistakes and accidents and mechanical failure; Syrians: in the hundreds or low thousands including civilians
Success Probability: Depends on the goal. It is high if the goal is simply punishment; much lower if the goal is regime change. Air wars alone rarely change governments.

3) Bomb and invade the country, topple Assad and leave immediately. This option is much more costly and will also result in US casualties. It also must be concluded within 90 days or so or incur the need to secure Congressional support, something that is not guaranteed. It also results in a power vacuum, chaos and potential all out civil war, from which only al Qaeda benefits. On the other hand, it has the potential of deterring Iran and forcing them to the bargaining table or it may just speed up their efforts to acquire nuclear weapons.

Cost: $50 - $200 billion minimum, and this is if it takes a month or less to accomplish the goal. This is based on the cost of the war in Iraq. The initial assault was extremely costly since war is done as a rapid, "damn the expense" pace.
Time: Assume one month regardless of outcome
Casualties: US, 50 - 100, plus 1000 wounded (roughly what happened in Iraq). Syrians, 10,000 killed and wounded including women and children
Probability of success: Short term, very high; long term, doubtful. In the aftermath there will be a free for all with the US, al Qaeda, the Russians and the Chinese trying to get a piece of the pie, not to mention Iranian interests

4) Bomb, invade and occupy. Like we are going to do that. George Bush wasted any possibility the US has to wage any kind of long term war. Not only is our military on a draw-down, we still have not replaced the equipments destroyed and damaged in Iraq and won't be able to for some time, especially under the sequester.
Cost: 250 billions minimum
Time: One year minimum
Casualties: US 100s - 1000s including killed and wounded; Syrians 10s of 1000s depending on their resistance.
Probability of success: What does victory look like? Once we leave civil war will break out and al Qaeda is best positioned to win, so US probability of success is low longterm

In the end, we need to define what success and victory looks like to the US. If is simply the establishment of a new government, regardless of what it looks like, we can quickly do that -- assuming the Russians and Iranians don't become involved. But if it means stability, a contained al Qaeda and democracy, then we will fail, just as we have failed in Iraq, a land where Iran rules and democracy is a joke.