A brief history of chemical weapons
This week's chemical weapons attack in Idlib Province, which has killed more than 60 people, is the worst and latest in a series of uses of chemical weapons against civilians in Syria.
In this case, symptoms reported among many of the victims, such as pin-hole sized shrunken pupils, point to the potential use of a nerve agent such as Sarin instead of - or possibly alongside - the more usual chlorine gas.
The Assad regime has long used chlorine gas cylinders within barrel bombs dropped on densely populated areas as a weapon in the urban sieges which have come to define the Syrian Civil War.
Chlorine is normally used as an industrial chemical, but when inhaled, it causes severe damage to the eyes, throat and lungs - fatal in sufficiently high doses, and the cause of lasting health problems at lower exposure levels.
However, it remains far less deadly than specialised nerve-gas agents such as VX and Sarin.
The use of poison gas as a chemical weapon in warfare dates back to the First World War, during which both sides deployed first chlorine and, later, the much more lethal phosgene and mustard gas from large cylinders and artillery shells - in an attempt to break the attritional stalemate of trench warfare.
At first, when used by the German Army in 1915, French troops fled en masse from clouds of chlorine, but German troops were reluctant to advance into their own gas clouds and gaps in allied lines were quickly plugged.
|Today's chemical weapons
disproportionately affect children [Getty]
From simple damp cloths mounted over the mouth and nose to more recognisable gas masks, countermeasures were soon developed by both sides and, as a result, the even deadlier phosgene and the extremely debilitating mustard gas were used in great quantities by both sides until the end of the war - but were unable to replicate this initial partial success.
Chemical weapons were not decisive during the First World War, partly due to the inherent limitations of the chemicals available at the time, and partly due to the rapid development of gas masks and training protocols - which enabled trained troops to remain effective when attacked with gas.
Gas also proved very hard to control and difficult to disperse reliably - it often blew back onto the trenches of the side which launched it, and indiscriminately killed civilians and animals behind the lines when blown beyond the immediate frontline area.
Although gas attacks caused relatively few fatalities among those exposed during the First World War itself, many of the victims were scarred for life both externally and internally, and died of complications relating to their exposure years after the armistice.
|Adolf Hitler had been gassed as a corporal during the First World War and so had a deep hatred for the weapons and forbade their use|
During the Second World War, all sides refrained from using chemical weapons, in spite of having significant stockpiles in their inventories.
Adolf Hitler had been gassed as a corporal during the First World War and so had a deep hatred for the weapons and forbade their use unless used by the Allies. The Allied powers did not think that any military effect which might be gained from its use would be worth breaking the mutual restraint being shown with regards to chemical weapons in an otherwise total war.
However, the Cold War saw the development of radically more potent chemical weapons than those which dominated the First World War.
The most well known and lethal of these agents are Sarin, Tabun, Anthrax and VX. While stockpiled by all the major powers during the Cold War, they first saw battlefield use in the Iran-Iraq war, during which Iraqi forces used Sarin, Tabun and modernised mixtures of mustard gas against Iranian "human wave" attacks to lethal effect.
Mustard gas and nerve gases were also used by Saddam Hussein against the Kurdish town of Halabja in 1988 after the town fell to Iranian and Kurdish forces, killing thousands of Kurdish civilians and creating an international outcry.
These are the only known large-scale uses of modern chemical weapons on the battlefield, and under the provisions of the Chemical Weapons Convention (CWC), Albania, India, Iraq, Libya, Russia, Syria, and the United States have committed to destroying their remaining stockpiles.
|The use of such chemical weapons, even in small quantities, as a tool of warfare also remains a war crime|
However, in spite of this, the Assad regime has repeatedly employed Sarin gas against civilian areas, most notably during the Ghouta attacks in August 2013, until a Russian-brokered deal to relinquish its entire stockpile as soon as possible was struck in September 2013 to avert US-led strikes against the regime.
Since then, attacks have continued but have tended to be limited to the less lethal chlorine or tear-gas cylinders contained within the ubiquitous barrel bombs which the Syrian Air Force drops on besieged areas on a daily basis.
The amount of chlorine dispersed by these "improvised" chemical weapon attacks on Aleppo, Hama and parts of Idlib province are not sufficient to achieve a large-radius of lethality.
In fact, the explosive power of the barrel bombs which these cylinders are usually dispersed by, is almost certainly more devastating to civilians than the gas dispersed.
However, in the case of this week's Idlib attack, Sarin may once again have been deployed by the regime, which is particularly significant since it comes shortly after the Trump administration clarified that the removal of Assad was no longer considered a high priority.
It also suggests that the complete destruction of the Syrian nerve agent stockpiles has not been accomplished. Both chlorine gas and particularly the much more lethal nerve agents such as Sarin are terrifying psychological weapons and particularly severely affect children.
The use of such chemical weapons, even in small quantities, as a tool of warfare also remains a war crime.
Justin Bronk is a Research Fellow in Military Sciences at the Royal United Services Institute in London. Follow him on Twitter: @Justin_Br0nk