There is a further option that has not been exhaustively examined: that of a multi-national stabilization force. A stabilization force is neither an intervention nor a peacekeeping tool: It has the military capacity of the former, but the intentions of the latter. It does not aim for regime change, but to stop a particular bout of killing and to prevent more. The deployment of such a force helped stop widespread slaughter by the Indonesian army in East Timor in 1999.
It's neither an easy option nor a silver bullet. Memories of the disastrous U.S.-led multinational force in Beirut in 1982, which ended ignominiously after bombings of the U.S. and French barracks killed 241 American servicemen and 58 French soldiers, still linger. The multinational force deployed to eastern Congo in 1996 to bring an end to massive violence and displacement illustrates the strengths and limitations of this approach. That force achieved its goal: The mere pre-deployment of the force in Entebbe, Uganda, got Rwanda to withdraw its forces back across its border -- but not before Rwanda's rulers killed tens of thousands of former rebels. Still, in both eastern Congo and East Timor, multinational forces probably forestalled a far-worse slaughter.
A stabilization force of this kind can't fight its way into Damascus. The Syrian regime, or at least the army, doesn't have to formally acquiesce to its deployment, but it does have to signal that it won't fight it on the way in. This was the case during the 2006 Israel-Hezbollah war, when France, India, Italy, and others sent troops into southern Lebanon as part of a cease-fire arrangement. Hezbollah never agreed to the presence of the force, but quietly sent signals to Paris that it wouldn't contest its deployment.
Why might Syria's forces hold back? First, it's a lot better than opening the door to aggressive attempts at regime change, if those measures start becoming more credible. Second, as in eastern Congo, the pre-deployment of such a force can change the army's calculation. As my Brookings Institution colleague Martin Indyk has pointed out, the Syrian army has no appetite for a confrontation with Turkish forces, and even preparation of a force could shift its psychology and its assessment of the choices it faces. Western powers can also help by increasing the economic costs on Assad's business-community supporters, by working with international financial institutions to stipulate that debt accrued under this regime should be considered "odious" -- a step that would mean debt incurred under this regime need not be paid back, setting out a deeply uncertain economic future for Assad's business-community supporters.
Who could lead such a force? Turkey has understandably equivocated about the option of using its army to help protect civilians or stabilize Syria. The risks Turkey faces are enormous -- but it might be more willing to see its army deployed as part of a multinational force with international authorization, spreading both the operational costs and the political risk. Gulf states such as Qatar and Saudi Arabia, which have strongly signaled their desire to support the Syrian opposition, would be more than happy to supply the financing.
Such a force would first have to try to win Security Council authorization. Turkey would have to be willing to make the first move, as the willingness of one country to take a leadership role is usually a precondition of authorization. That's already a tough step -- but winning support from China and Russia, veto-wielding members of the Security Council who have already rejected two resolutions targeting Assad, will be even harder. Is it doable?
The odds aren't as slim as one might think. Tactically, the right approach here would be for Turkey to make this proposal, not the United States. The Turks could seek support from its emerging-power friends both inside the Security Council (India, South Africa) and outside it (Brazil, Indonesia). The emerging powers collectively make a great deal of noise about using the Security Council as a tool to avoid force aimed at regime change. So let them lead an effort to do so.