Mattis is splitting hairs over Assad's chemical mass murder

Brutal chemical attacks of this kind, though they occur with depressing frequency, are not the Syrian regime's mainstay. Bashar al-Assad and its allies marshal violence more consistently by other means.

Their primary tool is airpower, where the pro-regime coalition, which includes Russia, has supremacy. The nature of the aerial campaign waged by Russian and regime forces attempts to make ordinary civilian life in rebel-held areas untenable. Civilian areas are targets as much as combatants. Markets are frequently bombed by regime planes, as are hospitals.

At the beginning of the month, a hospital in Hama - built underground specifically to keep it safe from airstrikes - was put out of action by aerial attack. That attack was deliberate. It likely required specialised weaponry designed to break through up to 60 feet of rock. This is the profound challenge those who provide basic services in rebel-held Syria face. 

Despite what appears a recent escalation in violence in the farcically named 'de-escalation zones', strikes of this kind are a constant feature of the war. They are not something likely to end. The regime forces will not relent.

This brutal aerial campaign, because it is an essential feature of the conflict, does not arouse burning international outrage. As observers have become used to the pattern of violence, they have become inured to its sights and to its effects.

It helps no one that the United States has now decided not to punish comparable outrages because the weapon used is not nerve gas

When Russian and regime aircraft carried out what some have estimated to be hundreds of airstrikes in Idlib on 4 February, in apparent retaliation for the downing of a Russian plane by rebels, this fitted into the normal pattern of things.

That a Russian plane had been shot down – that was the story - was something new and different.

It takes rarer and more horrific events to attract global attention to the fate of Syria's civilian population. The apparent use of chemical weapons remains just rare enough, and suitably horrific, to have that effect.

Reports of chemical weapons use are always contentious. It's a visceral subject; these attacks generate emotive evidence - footage of incomprehension turning to fear, of children suffocating, followed by images of lifeless bodies, some of whom appear eerily unmarked, as if felled by an unseen hand.

When a chemical attack is alleged, initial reports can be frustratingly unclear, the early evidence so raw, the use of chemical weapons so stigmatised and criminal that there is a collective rush to deny responsibility and to attribute blame. 

Despite the chaos, it seems likely the regime has used chlorine gas several times in the last week. While these attacks may serve as outliers amid the general carnage, this is not an aberration; instead, chemical weapons use by the regime remains the norm, even after the United States undertook retaliatory action to punish the former's use of sarin gas, a nerve agent, in April last year.

They are both chemical weapons and comparable in terms of the horror and suffering they produce. But in terms of practical policy, a different reaction occurs when the Assad regime uses sarin and chlorine gas.

Read more: US believes deadly sarin gas could have been used in recent Syria attacks

This is a distinction the United States seems set to maintain. The American secretary of defence, James Mattis, has effectively separated the two, acknowledging that the regime has used chlorine but maintaining that what the US really worries about is more sarin use, a weapon the regime is also accused of having used in recent days.

Referring to the regime's use of chlorine, Mattis said, "We are even more concerned about the possibility of sarin use." The United States at the same time talked down the use of one chemical weapon while threatening the regime if it used - or was found to have used - another.

This is an artificial distinction, and maintaining it has only destructive consequences, not only for the Syrians who continue to choke on poisonous clouds, but for all concerned.

The Syrian conflict has brought hair-splitting to mass murder. Though hundreds of thousands of Syrians have died violent deaths in battle or have been killed in regime prisons or have succumbed to starvation, the use of chemical weapons, particularly the use of sarin, always seemed beyond the pale.

It was the arbitrary act the world community decided to monitor, the line the regime would not be allowed to cross.

When it was crossed in August 2013, the red line proved to be nothing of the sort.

First, the British House of Commons voted against action proposed by the then-prime minister, David Cameron. Then the Americans refrained from punitive measures. Barack Obama, who was president at the time, latterly numbers the decision not to punish the Assad regime's chemical war crimes among his proudest.

President Trump reflexively enforced that red line in April last year, after the regime again used sarin to attack Khan Sheikhun. But it helps no one that the United States has now decided not to punish comparable outrages because the weapon used is not nerve gas.

The American strike on the al-Shayrat airbase was widely welcomed. Other nations, including France, suggested that if chemical weapons were again used by the regime, they would conduct similar punitive measures themselves.

Very briefly, it looked as though the regime would be prevented from making use of its chemical arsenal, or at the least it would be punished severely if it ever did so again.

Unless the Americans act against the regime for its current and continuing use of chlorine, which seems unlikely, the nominally strong stance the new administration has taken on chemical war crimes will be for nought.

It takes rarer and more horrific events to attract global attention to the fate of Syria's civilian population

And worse than that, it will convince many, in retrospect, that even the decisive action of last April had been undertaken with partiality and insincerity.

War crimes must be punished, or they will merely be repeated, and what once inspired rare horror in Syria may become a constant presence, used in plain sight before an increasingly indifferent world.