Even with Syria's overnight decision to allow weapons inspectors access to the site of the recent chemical weapons attack, events in Syria appear to be sliding inexorably to some form of US military response.

The emphasis on the Mediterranean-based US Sixth Fleet appears to suggest that ship-based missile attacks of limited scope and intensity against select targets may be at the top of Obama's military response options. Although every fibre in President Obama's body tells him to steer well clear of military intervention in Syria, his stated 'red line' over the use of chemical weapons means that the credibility of the US is on the line.

However, unlike his predecessor, George W Bush, Barack Obama is only too aware of the limits of US military power to resolve complex situations. And Syria is complex.

The challenges facing any military response are set out in two recent letters from the Chairman of the Joint Chiefs of Staff, one to the Chairman of the US Senate's Armed Services Committee outlining the military options and their associated risks, and one to Representative Eliot Engel, the ranking Democrat on the House Committee on Foreign Affairs, which explains how the US military can shift the military balance in Syria, but which also says that there is no one in whose favour it should be shifted.

This latter point is the most telling, and explains why any military response to the chemical weapons attack is likely to be limited.

Obama will not be able to operate with a UNSC resolution, as it is nearly certain to be blocked by Russia and China. He will therefore need to ensure that he has as much international and in particular regional diplomatic backing for his actions as he can muster. He will also need incontrovertible evidence of Syrian regime complicity at the highest level for ordering the chemical attack, and he will need to ensure that he avoids civilian casualties.

On top of all of this, he will want to avoid damaging the Syrian military too much and thereby handing a military advantage to the opposition, few of whom have interests or values that are in line with those of the US. His response, therefore, is likely to be limited to targeting military equipment, preferably that which has a link to the use of chemical weapons, such as delivery platforms and command centres.

Syria has already commenced an information operations campaign, with Syrian state television claiming that Syrian troops have entered rebel tunnels in the region and have found where chemical munitions were stored. Both the Iranian Foreign Ministry and to a lesser extent the Russian Foreign Ministry have pointed the finger at rebel forces. Recent media reports in the region have tried to place responsibility on certain regime figures for the attack, possibly providing a way of localising responsibility in the event weapons inspectors are able to establish that the regime was responsible for the attack.

Over coming days we will see whether President Obama is able to achieve any certainty regarding the nature of the chemical weapons attack, who was responsible for it and who authorised it. The first two pieces of information will probably be ascertained to the public's satisfaction, but the last will depend on intelligence, which will be uncertain at best and probably never made public.

For a president who has made the ending of two wars his signature foreign policy achievements, Obama has been disinclined to engage in another, even in the face of loud and partisan calls for action. A limited missile attack would fit well with President Obama's desire to avoid decisive military engagement in Syria while not ignoring the use of chemical weapons. It would be punitive without being decisive, it wouldn't materially assist the armed opposition, it wouldn't require the type of force buildup or supporting attacks against air defences needed for a sustained air campaign, it would give some substance to his 'red line' and it would allow a response to be graduated if more CW attacks were carried out.

Obama has no options without some significant downside, so a limited missile attack may prove to be the least worst response.

Photo by Flickr user Official US Navy Imagery.