The probability that the United States will make a single military reponse to the chemical-weapons assault near Damascus is very high.
President Obama warned on 20 August 2012 that a major chemical-weapons (CW) attack by Bashar al-Assad's regime on Syrian civilians would cross a “red line”. With the evidence of civilian casualties in the eastern suburbs of Damascus on 21 August 2013 such an attack now looks to have happened, though Obama is being very cautious in his response. In part this may stem from a lack of conclusive evidence about the perpetrators of the attack; but it also reflects the changing character of the war.
An important aspect of the change is the increasing influence of the more extreme jihadist paramilitaries. This raises the prospect of an Islamist "safe zone" being established in northern Syria in the wake of the fall of the regime. Such an outcome would be unacceptable to the United States and its allies.
Some US sources see the current situation in Syria - which can be described as a violent stalemate, where neither regime nor extremist rebel groups can gain the ascendancy - as something Washington could live with in the long term (see Edward N Luttwak, “Keep Syria in a stalemate”, International Herald Tribune, 24 August 2013). The appropriate policy conclusion would then be to arm “acceptable” rebel forces to a degree that would prevent their defeat by the regime, but not to intervene in any way that would ensure regime capitulation.
The chemical challenge
How these calculations relate to the most recent chemical-weapons attack is problematic. Any large-scale intervention by the United States, for example a sustained air assault on Assad's military forces, would be unpopular domestically and fraught with dangers of escalation. This explains why senior US military commanders are very cautious about taking a step of this kind. In any case there is no United Nations mandate for such action and any attempt to secure one would be vetoed by Russia.
At the same time, if Obama does nothing he can be portrayed as weak and (in light of his "red line" statement, inconsistent - charges that the president's Republican opponents will utilise to maximum possible political advantage.
In addition, the latest CW assault carries three direct dangers, all of which stretch beyond Syria's borders. The first is that, without a response to his action, Assad could escalate CW use. This would be of comparatively little military significance if the regime was facing conventional forces equipped with protection from chemical and biological weapons (CBW), but it could be highly effective against rebels in urban areas - not least as it is a form of terror spreading fear and puncturing morale, as well as tying down limited medical resources. These weapons could even affect the direction of the war as a whole.
The second danger is that repeated CW attacks would do serious damage to the international prohibition on chemical weapons, one of the few arms-control successes of recent decades. The chemical-weapons convention (CWC) was signed in 1993 and has been in operation since 1997; it has 187 parties and a substantial verification system, though the few non-signatories include three states in the region - Egypt, Israel and Syria.
The third danger is that repeated deployment and use of CW in Syria would greatly increase the risk of CW being obtained by radical paramilitaries, either through interdiction of supply-chains or the acquisition of unexploded ordnance.
The military calculation
In light of all this, the Obama administration may therefore decide on a very specific response in the form of a single punitive attack, while warning the regime that any further use of CW would trigger further action. The legal basis for such a course would be akin to that which underpinned intervention in the war over Kosovo in 1999. There, Washington and its allies lacked UN approval but justified an extensive and ultimately effective seventy-eight-day air war on Serbia principally on the basis that they were acting to protect a vulnerable civilian population. A similar justification could be used in Syria (see Mark Landler & Michael R Gordon, “Air War in Kosovo Seen as Precedent in Possible Response to Chemical Attack”, New York Times, 24 August 2013).
If this choice is made, it could be implemented very quickly - the most likely timescale is over the next seventy-two hours (i.e. 24-26 August), and the most probable option an individual strike using a salvo of sea-launched Tomahawk cruise-missiles aimed at one target of importance to the regime. The latter would not be directly related to the regime’s chemical arsenal, because that would risk releasing CW agents. Rather, the target would most likely be a major intelligence or state-security centre or a command-and-control facility close to the heart of the regime in Damascus.
Syrian security planners will have thought this through, and taken steps to limit the vulnerability of any such targets. Their US counterparts will in turn have anticipated the Syrians' preparation, and may be content to concentrate on communications facilities that cannot be hardened or evacuated at short notice.
In any case, the use of military power against the Syrian regime would primarily be an exercise in symbolism, with its actual physical effect a secondary matter. The purpose would also be to leave the regime unsure as to the nature of any response to further CW use.
There is currently a high probability of such an attack.
It too would carry dangers and uncertainties. If Russia is given advance warning, this may have a limited impact on currently poor US-Russian relations in the short term; but in the longer term, attitudes might harden further. In Iran, a US operation could weaken the new president, Hassan Rowhani; any US attack related to any WMD facility in the region will tend to strengthen Tehran's more hawkish elements.
In addition, any direct external intervention - especially in the raw form of military power - entails many potential unexpected consequences. The fact that Syria's conflict has strong elements of a complex proxy war, the aftermath of even a single punitive and largely symbolic action is unpredictable. Just how unpredictable may soon become clear.