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The attempted assassination of Sergei and Yulia Skripal has sparked an international crisis, with UK allies expelling scores of Russian diplomats in solidarity against the apparent use of chemical weapons on British soil. But behind the scenes, another crisis is unfolding: the first ever test of whether the international treaty banning these weapons can be used in a world for which it wasn’t designed.

This matters much more than a few diplomats being sent home. Arms agreements backed by science, like the 1997 Chemical Weapons Convention, are a centrepiece of the “”&����������;