The use of chemical weapons in Syria threatens to wreck one of the most successful examples of international cooperation. Step-by-step work over the years has resulted in at least 80% of the world's known stockpiles of chemical weapons being destroyed, and efforts are continuing to destroy most of the rest.

To date, only three countries – Egypt, North Korea and South Sudan – have neither signed nor ratified the Chemical Weapons Convention, plus Israel which has signed but have not ratified. Having got so close to eliminating them entirely, the world can't afford to let anyone start normalising their use.

Over several years, however, the Assad regime has repeatedly used chlorine as a chemical weapon and, on at least two well-documented occasions, the nerve agent, sarin. 

Not only that. In 2013 Syria renounced chemical weapons and became a party to the Chemical Weapons Convention but since then there has been growing evidence that it is cheating. Its initial declaration to the Organisation for the Prohibition of Chemical Weapons (OPCW) was incomplete and further investigation by the OPCW has been met with stonewalling.

Throughout this period, Syria's ally, Russia has been propagating conspiracy theories aimed at diverting attention from the regime's misbehaviour. Russia, of course, also faces accusations of carrying out a nerve agent attack in Britain – to which it reacted in a similarly disingenuous fashion.

If chemical weapons are not to become normalised there must be an international response. The world – not just the United States – must show the Assad regime that its behaviour is unacceptable. Something needs to be done but the difficult question is what.

Last year, in retaliation for the sarin attack on Khan Sheikhoun, the US launched dozens of cruise missiles against a Syrian airbase – apparently with little effect. The upshot of that, as Sam Heller points out in an analysis for the International Crisis Group, is that any military action taken now would have to be "more robust" than the action taken in 2017, "while also being accompanied by clear communications and concrete demands that Damascus and its allies could understand and reasonably implement".

This, as Heller says, would present "a considerable challenge":

"Not only would it be difficult to organise and message a multilateral intervention in a complex political and military space, but it is uncertain exactly how much damage a strike would have to inflict to achieve the desired goal and effectively deter the Syrian government from further chemical weapons use, while avoiding a dangerous Russian response or other form of escalation. In essence, a narrow, limited attack is unlikely to deter repeated chemical weapons use, while a broader one could have unintended and uncontrollable consequences."

An article by Justin Bronk for RUSI, the British-based military/security thinktank, is equally sceptical and concludes that the military options have "serious limitations":

"When all is said and done, the US, France and potentially the UK have the means to undertake another round of symbolic but ultimately ineffective strikes using standoff weapons against Assad’s airbases and potentially weapons storage areas. However, this is unlikely to affect the regime’s determination or ability to continue its military efforts to crush the opposition using urban sieges, massive bombardments and chemical weapons.

"A larger effort to annihilate the Syrian air force or even the regime’s ability to continue the war effectively would require a large-scale air campaign which could be only conducted if it involved the suppression – by force – of the Syrian air defence network and possibly the Russian military’s air defences. This would by necessity involve killing Russian troops and, for this reason, is an extremely dangerous and unlikely course of action."

On a different tack, an article by Dan Kaszeta, a London-based consultant in security and chemical defence, looks at practical options for impeding, if not necessarily preventing, further use of chemical weapons in Syria.

There is no magic bullet, Kaszeta says, but "a combined multidisciplinary effort" could have some effect. This would involve tackling various aspects of Syria's chemical warfare infrastructure: development of weapons, supply chains, production, transport, delivery means, and personnel. 

But in each of those areas there are difficulties. Efforts could be made to stop Syria acquiring hexamine – an essential additive in the regime's formula for sarin – but keeping chlorine out of the regime's hands would be almost impossible because it has many legitimate uses, such as water purification. 

It may be too late to have much impact on the regime's use of chemical weapons but there are still ways of discouraging their use in future by others. Kaszeta writes:

"People make these chemical weapons and load them onto aircraft or into rocket launchers. People transport them. People fire them or drop them. It is a well-established principle in war crimes prosecution that following orders is not a defence. No person having any part in the infrastructure from production to use against a target should be immune to penalty.

"For example, intelligence efforts should be undertaken to determine the identity of pilots flying chemical strikes.  A number of names of people associated with the chemical warfare program are already documented and specific sanctions should be made. A scientist with ties to the SSRC [the Syrian Scientific Studies and Research Center] should never get a visa to travel. Eventually, this conflict is going to end. War crimes prosecutions must take place."

That legal process should also focus on Assad himself. He cannot be allowed to be part of any political settlement and should eventually be indicted for chemical and other crimes.

Interestingly, that may be becoming a more realistic proposition than it has been up to now. Writing for the Emirati newspaper, The National, Hassan Hassan – co-author of a bestselling book on ISIS – argues that the Syrian regime is in its strongest position for several years and, paradoxically, that could make Assad dispensable:

"No rebel forces are capable or even willing to resume active fighting against the regime. More importantly, none of the countries involved in the Syrian conflict is interested in seeing that happen. Turkey is already tied to mutually beneficial deals with Russia throughout the north. The Gulf states have almost completely pulled out of the conflict at least since 2016. The US is also not interested in destabilising the regime, certainly not in a way that favours the rebels. American officials view the rebels as either too radical or too disorganised to ally with them. And the regime knows this all too well.

"Which brings us to an intriguing argument shared by some fellow Syrians, including some who lean toward the return of the government and are critics of the opposition. Because the survival of the regime is more assured today than at any point in recent years, the argument goes, Bashar Al Assad is now more dispensable than when his symbolism was perceived as essential for his camp. Mr Al Assad can go and the Syrian government can still survive, a formula that the same Syrians would not have accepted a year ago."