US President Barack Obama has moved a step closer towards direct intervention in Syria with his statement that there is now evidence that chemical weapons have been used in Syria’s civil war. Obama has previously said the use of chemical weapons would be a “red line” that, if crossed, would trigger US intervention.

However, Obama has said it is not yet absolutely clear who was responsible for the use of the chemical weapons, and that it is critical to clarify this point so as to ensure international support for US intervention. His caution reflects growing concern over both the mounting death toll within Syria as well as the inexorable drawing in to the conflict of outside forces, in particular Lebanon’s heavily armed Shiite militia Hezbollah.

Both the Bashar al-Assad government and the Syrian opposition claim chemical weapons have been used in the conflict, as recently as last Sunday. This supports earlier Israeli claims chemical weapons were being used in the Syrian conflict. There has been a high level of reluctance to take such claims on face value, however, given the disrepute of similar claims that rationalised the start of the Iraq War. Even if it can be established who has used chemical weapons — thought to be the nerve gas Sarin — it is not yet clear what form intervention might take, much less the shape of international reactions to such an intervention.

With the US public weary over the Iraq and Afghanistan wars, it is unlikely that the US would commit ground troops to Syria. A belligerent response from Syrian ally Russia, and to a lesser extent Iran, are also factors against a ground intervention.

However, a bombing campaign and related air cover, as in Libya in 2011 and in Yugoslavia in 1999, have been shown to be effective in either changing the course of a ground war or compelling a government into submission. With the Assad regime only slowly losing ground in its now two-year-old civil war, such an intervention would be likely to tip the outcome against his government forces.

One factor complicating of any hastening of the fall of the Assad regime is that Syrian opposition forces are now deeply divided. The Free Syrian Army is supported by the US and its European allies, and the explicitly al-Qaeda-affiliated Al Nusra Front is supported by Saudi Arabia and Qatar. Although it wants the Assad regime to go, the US and its allies are deeply opposed to an Al Nusra takeover of Syria. A civil war between Al Nusra and the FSA is also seen as increasingly likely following the fall of the Assad regime.

On-ground intervention by Hezbollah, which is supported by Iran, has led Al Nusra leaders to say that, following the fall of the Assad regime, Hezbollah’s destruction will be the next priority. Contemplating a possible Al Nusra takeover in Syria and a widening of the war into Lebanon and possibly Iran, the US is focusing on how its increasingly likely intervention could shape Syria’s highly contentious future.

*Professor Damien Kingsbury is director of the Centre for Citizenship, Development and Human Rights at Deakin University