Shashank Joshi: How serious is Syria's threat to use chemical weapons?

You might remember seeing this movie: ragtag rebels, with obsolete weaponry, battered into near-submission by a far superior army. But, as defections take their toll and foreign support rolls in, the tide turns. As in Libya, so in Syria – even if it's taken the latter's insurgents far longer to arrive at this phase.

Defections have reached 100 soldiers a week, with the rate accelerating after last week's devastating bombing of top regime figures. The regime's death rate now exceeds 100 a day, in part as a result of growing rebel proficiency in the use of improvised explosive devices.

The irony is that for the regime, this is a lethal taste of its own medicine. The proliferation of IEDs – in many cases planted by the same Sunni militants sent into Iraq by the Assad government only a few years ago – is suppressing the mobility and morale of an already haemorrhaging army. As growing patches of territory become no-go areas, the rebels can consolidate further.

As the regime is worn down, how should we interpret its threat to use chemical weapons only against "external aggression"? Although there's room for ambiguity, given that the rebels are denounced as foreign terrorists, it's fairly clear that this refers to conventional military intervention. Assad, like everyone else, will have read the reports of US co-ordination with Israel and Jordan to, respectively, bomb or seize Syrian chemical weapons. As such, the regime's signalling is understandable. It seeks to deny the West a reason for intervening, and prohibitively raise the cost of doing so.

The threat cannot be dismissed. Whereas Iraq did not have active chemical weapons in 2003, despite the frantic efforts of Saddam Hussein's son Uday, Syria has now admitted to possessing them. Damascus likely has a large and diverse (and legal) stockpile.

Yet, there are reasons for scepticism. Is it realistic that a Syrian commander would follow orders to hurl VX gas against, say, Turkish forces, in defence of an obviously crumbling regime, and when the former head of Syria's chemical weapons programme has defected? It seems unlikely, particularly when the battlefield effectiveness of such weapons is anyway so limited.

Nonetheless, the fog of war means that the regime's intentions will be unclear. What if an accident during transportation, perhaps even caused by an IED, is confused with deliberate release? And what if rebel forces outside the ambit of the Free Syrian Army overrun a unit carrying chemical weapons? Finally, Assad would do well to remember: you can't fire mustard gas at an F-16.

Shashank Joshi is research fellow at the Royal United Services Institute