The attack in Britain on the Russian former double-agent Sergei Skripal and his daughter Yulia marks, so far as anybody knows, the first offensive use of a nerve agent on the territory of a North Atlantic Treaty Organization member. The response needs to be both firm and multinational.
British Prime Minister Theresa May's statement to a hushed parliament this week conveyed the seriousness of the act, and was clear about where the blame lies: There's "no alternative conclusion other than that the Russian state was culpable," she said. There are limits, though, to what the U.K. can plausibly do on its own. It should do what it can -- while calling on allies to join in a concerted response.