Why would the West respond to the latest Syria chemical attack?

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Northern Syria (CNN)Only twice before has the use of chemical weapons brought to Syria the threat or reality of US strikes -- in Ghouta in 2013, and in Khan Sheikhoun in 2017. Both occasions involved the alleged use of sarin gas, a nerve agent banned under the 1993 Chemical Weapons Convention.

In 2013, the use of the gas, reported by United Nations investigators, crossed one of then-President Barack Obama's red lines, yet no military action came. Instead, the Organization for the Prohibition of Chemical Weapons (OPCW) went into Syria to monitor the destruction of the country's chemical weapons program.
They were on the whole successful. In October 2013, the OPCW confirmed "the government of the Syrian Arab Republic has completed the functional destruction of critical equipment for all of its declared chemical weapons production facilities and mixing/filling plants, rendering them inoperable." Yet some of the sarin stockpiles remained.
    One Western official told CNN the regime could not surrender the sarin gas used in the Ghouta 2013 attack, as that would be like handing over a murder weapon in a killing to which you had pleaded not guilty.
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    This may have enabled the April 4, 2017 attack in Khan Sheikhoun. It appears the shocking video of children and civilians choking to death, foaming at the mouth, caused the US to decide sarin was being used.
    US President Donald Trump launched 59 Tomahawk missiles in response just two days later -- a very swift turnaround considering that any samples would have to have been rushed out of a war zone and tested. The OPCW only declared sarin had been used 13 days later, on April 19.

    What happened this time in Douma?

    No samples have been delivered yet from the site where activists say two chemical attacks struck Saturday, partially because Douma is the last rebel-held town and otherwise under siege. The gas appears to have sunk into basements -- similar to the attack in 2013 -- where children and families were sheltering. The second attack killed the most.
    The Syrian American Medical Society (SAMS) said emergency workers on the ground reported that patients showed "respiratory distress, central cyanosis, excessive oral foaming, corneal burns, and the emission of a chlorine-like odor.
    This image released early Sunday, April 8, 2018 by the Syrian Civil Defense White Helmets, shows a child receiving oxygen through respirators following an alleged poison gas attack in the rebel-held town of Douma, Syria.
    They added: "Symptoms indicate that the victims suffocated from the exposure to toxic chemicals; most likely an organophosphate element." Omar Ibrahim, a SAMS doctor who treated the Khan Sheikhoun victims, told CNN: "I do believe it is the same gas used last year in Idlib, sarin gas. I saw the photos and videos coming from Douma ... victims had similar signs and symptoms."
    The World Health Organization (WHO) said partners told them 500 people were affected and that "there were signs of severe irritation of mucous membranes, respiratory failure and disruption to central nervous systems of those exposed." More than 70 died in basements they said, 43 of them from "symptoms consistent with exposure to highly toxic chemicals."
    Hamish de Bretton-Gordon, a chemical weapons expert who used to lead NATO's response team, said the symptoms he had seen suggested this could be a nerve agent but that it was hard to be sure without samples.
    He added, however, that the number of casualties -- said to be 500 affected, according to WHO, and 43 dead from the gas -- was too high to be attributable to a more commonly used gas like chlorine. This leaves open the idea that it could have been a mixture of substances, he said.
    The smell of chlorine with the effects of a nerve agent are a particularly troubling development, suggesting perhaps some new cocktail designed to appear like the less-harmful chemical used repeatedly.

    Nations seem willing to act together

    There have been multiple instances of a chlorine-like or other gas being used in the past. In 2016, the OPCW concluded two female casualties "reported to have been involved in the incident in Um-Housh, Aleppo of 16 September 2016 were exposed to sulfur mustard."
    Similarly in August 2015, biomedical samples taken from Marea, north of Aleppo, showed "at least two people were exposed to sulfur mustard and were in the process of recovering from the exposure. It is additionally very likely that the effects of sulfur mustard resulted in the death of a baby."
    They also concluded that in Idlib province between March 16, and May 20, 2015, various incidents "likely involved the use of one or more toxic chemicals -- probably containing the element chlorine -- as a weapon."
    When it appears that nerve agents have been used, the West's response is very different -- and it is worth noting that the United States is not acting alone here. As we have seen since the use of the Novichok nerve agent in the attempted murder of former Russian spy Sergei Skripal and his daughter, Yulia, several nations appear willing to act in concert to protest the use of chemical weapons.