A year and a half ago, a colleague made two predictions about events in Syria—which reflected the conventional wisdom in many Western capitals. The first was that the anti-Assad rebellion had reached critical mass, and that continued defections from the Syrian military and the establishment of liberated zones in Syria meant that the regime's days were numbered. The second was that the Russians would not maintain their opposition to Bashar al-Assad's removal from power; that Moscow, unwilling to jeopardize its "reset" with Washington and its close economic ties with key Western European states, would scramble for a face-saving solution.

Neither of these predictions has panned out. The Assad government has not crumbled, and its regional allies have rallied to aid its survival, with Hezbollah and Iranian Revolutionary Guard fighters now reportedly deployed to bolster Assad's forces. The opposition remains unable to forge a more unified and effective front. And Russia has moved from casting vetoes in the UN Security Council to rearming and re-equipping the Syrian military. If the S-300 anti-aircraft systems are delivered and installed, Damascus would be in a better position to raise the costs for the United States and its European allies should they desire to create a Libyan-style "no-fly zone" over the country (although questions remain about how soon the S-300s could be truly operational).

The preferred Russian outcome for Libya (which did not materialize once Operation Odyssey Dawn began) and Syria has been a grand bargain, a power-sharing arrangement between rebels and the regime that would keep the existing rulers from being forcibly overthrown. However, given the results of the recent meeting of the Syrian opposition in Istanbul, the early signs for the success of the so-called "Geneva 2" meetings to try and find a diplomatic solution to the civil war are not encouraging.

But the Russian gamble may be that if Moscow raises the costs for Western action in Syria, this, along with recent battlefield reversals for the opposition, may make the opposition more amenable to talk. The Russian experience in Chechnya—where former rebels ended up becoming key components in the Russian power structure for the Caucasus—suggests that they expect that some of the groups currently in the opposition could, at some point, be co-opted into a new government of national unity. Or at least they might be willing to accept leaving parts of the Assad regime in control in Damascus in return for effective autonomy for their particular areas of the country.

Washington now has a very unpleasant choice to navigate. Without more direct Western assistance, beginning with arms shipments and possibly including the use of air power, the rebellion will be stalemated and its gains possibly even reversed. But the S-300 missiles could be a game-changer, by altering the calculations about the risks involved in using air power over Syrian airspace. Unlike previous Soviet- and Russian-supplied components, which were destroyed by the Israelis in the 1982 Lebanon war or which proved insufficient to stop Israeli strikes in both Lebanon and Syria (including the campaign to destroy an alleged nuclear site in 2007 and the most recent raid earlier this year to destroy an arms shipment meant for Hezbollah), the S-300 is seen as more capable and effective. (However, it should be noted that the actual capabilities of the S-300 against what the Israeli or U.S. air forces could put into the field are unknown.)

The Russian announcement fits Moscow's long-standing position that the "international community" ought to be mediating a ceasefire and political settlement between Assad and the opposition, rather than having the West repeat the 2011 Libya operation to use its air power to tip the balance on the battlefield in favor of the rebels. The S-300 system, as well as other components ordered by Damascus, are seen by Russia as part of a "fence" that tells the West not to consider direct military action. It is designed not only to protect Russian equities in Syria, but, as I have argued on other occasions, is an important signal being sent to other Russian partners, particularly in Central Asia, that Russia will defend Westphalian principles of state sovereignty for its friends against the precepts of Western humanitarian intervention.

But there are risks as well. Despite Russian sympathy for the Serbs during the Yugoslav wars of secession, Moscow did not provide military equipment; and when NATO units began strikes on the Serbs, particularly during the 1999 Kosovo campaign, Russia did not rush arms to Belgrade to be used to hold off Western air or ground forces. (And the Russian diplomatic role in convincing Slobodan Milosevic to accept terms may have been the source of the optimism in more recent times that Russia would somehow convince Assad to step down from power.) If, however, the S-300s are operational and the United States does begin efforts to create and enforce a no-fly zone, then for the first time since the Cold War, Moscow would be supplying weapons that could be used to target U.S. forces. What makes this development particularly problematic for the U.S.-Russian relationship is that Syria's existing stockpile of Russian-supplied equipment presents a challenge but not an insurmountable obstacle for any potential U.S. operation. But if the Russians engaged in the ongoing supply and upgrade of Damascus's air defense capabilities—with the express purpose of thwarting U.S. intervention—it would be very difficult to compartmentalize this and prevent it from contaminating other areas of U.S.-Russia relations.

But Washington can no longer assume that Russia will automatically back down. Having refrained from using their veto to block the Libya operation, and having broken the contract to supply the S-300 system to Iran, the Russians may be eager to see what concessions they might extract from the United States not to go through with the delivery of the S-300s to Syria.

Fortunately, there is still a window for maneuver. A more careful perusal of Assad's statements on the S-300 suggests that the system has not been actually delivered to Syria—and it cannot be set up in a flash. It takes time to ship, set up, deploy all the components and train personnel who can effectively use it. The S-300s would not be operational until at least November (assuming deliveries of components have begun), and it may take up to a year or more to be fully operational.

Senator John McCain wants the United States to act now to support the opposition. Secretary of State John Kerry wants to give a chance for the diplomatic process to work. But Washington no longer has an unlimited amount of time to ponder its course of action. Moscow's announcement has started the clock.

Nikolas K. Gvosdev, a senior editor at The National Interest, is a professor of national-security studies at the U.S. Naval War College. The views expressed are entirely his own.