Within hours of the gas attack on the Syrian town of Khan Sheikhoun, the conspiracy mongers were out in force.

They claimed that nothing had happened and the whole incident had been a stunt, or perhaps the town had gassed itself in a bizarre act of self-immolation, or someone – anyone – was responsible for the atrocity other than Bashar al-Assad’s regime.

This episode last April showed the convergence of two scourges of our age: the use of chemical weapons and the proliferation of fake news designed to hide the guilty.

There is only one rightful response to this pernicious combination. The international community must be able to uncover the truth by means of independent inquiry, empowered not only to say whether chemical weapons were employed but by whom.

And it is that vital power to attribute responsibility for chemical attacks in Syria that British diplomacy has just helped to restore.

Our efforts became necessary because of what followed the Khan Sheikhoun attack. In October 2017, a joint investigation by the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) found that one of Assad’s military aircraft had dropped a bomb laden with sarin nerve gas on Khan Sheikhoun (contrary to the energetic outpourings of the conspiracy theorists).

This was the fourth time this joint investigation had named the Assad regime as responsible for chemical weapon attacks in Syria. The terrorists of Daesh were held culpable for two other incidents.

But Russia chose to respond by protecting Assad and vetoing the renewal of this investigation in the Security Council last November. We then entered a period when the OPCW’s experts were able to investigate chemical attacks in Syria, provided they did not say who was responsible.

They were effectively asked to indulge a fiction whereby chemical weapons might descend from the sky of their own volition, without any agent or perpetrator. It was as if a vow of omerta had to surround the identity of the guilty party.

The OPCW reported that chemical weapons had been used twice more in Syria – in the towns of Lataminah in March 2017 and Saraqib in February this year. But they did not identify the perpetrator.

I sensed that a new and profoundly damaging taboo was slowly emerging, a taboo that applied not to the use of chemical weapons but to naming whoever was responsible. It was as if the real offence was not killing people with poison gas, but daring to identify the perpetrator of such wickedness.

Hence the importance of the change that was achieved this week. On Wednesday, a special conference of states parties to the Chemical Weapons Convention adopted a British-drafted “Decision” to allow the OPCW to make full use of its powers to attribute responsibility for chemical attacks in Syria.

If you had joined me in The Hague, you would have shared my pride in the British diplomats who were lobbying scores of countries, convincing the waverers and countering the frantic efforts of Russia and Iran to scupper the plan.

Our network of embassies threw itself into this campaign, working alongside countries across the world. I spent much of the day meeting or calling dozens of my counterparts. In the end, our proposal carried the day by 82 votes to 24 – a better result than we had dared hope.

Thanks in no small measure to British diplomacy, the OPCW will be able to answer all the vital questions about any future incident: what happened when and where – and who was responsible.

We are placing the taboo back where it belongs: over the singular horror of using chemical weapons. If any such attacks happen again, there will once more be international investigators empowered to discover the culprit. You can be proud of the British diplomats who did so much to bring this about.

Find out more about the international community coming together to strengthen the global ban against chemical weapons use.

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