By Editorial Board,

THE EUROPEAN UNION and Britain have now delivered a reminder notice to the Kremlin, saying, “The use of chemical weapons constitutes a serious breach of international law.” The global treaty banning chemical weapons was signed by Russia in 1993 and ratified by Moscow in 1997. So it is entirely appropriate that sanctions be imposed on Russian officials for the poisoning with a chemical agent of opposition leader Alexei Navalny. The United States has yet to act, and must not delay.

Mr. Navalny, the leading opposition figure to President , survived an assassination attempt in August after political organizing in Tomsk. He fell ill aboard a plane en route to Moscow; the plane made an emergency landing in Omsk. Recovering in Germany, where he was treated, Mr. Navalny has blamed Mr. Putin. The Organization for the Prohibition of Chemical Weapons (OPCW) has confirmed that the substance used to poison him had “similar structural characteristics” to the Soviet-era nerve agent Novichok. In response to this, Russia’s reaction has been to deny and obfuscate rather than investigate.

The attack on Mr. Navalny, an anti-corruption fighter who has demonstrated remarkable resiliency in leading the opposition, must have been approved at the highest levels. Novichok was a chemical weapon created by the Soviet Union near the end of the Cold War, and the OPCW report suggested the substance used to sicken Mr. Navalny may have been modified in some way from earlier compounds. Novichok was also used to poison former military intelligence officer Sergei Skripal and his daughter in 2018 in England.

These look like state-sponsored assassination attempts. As an E.U. statement said, Novichok “is accessible only to State authorities in the Russian Federation. In these circumstances and taking into account that Alexei Navalny was under surveillance at the time of his poisoning, it is reasonable to conclude that the poisoning was only possible with the involvement of the Federal Security Service.”

The new sanctions — asset freezes and travel bans — are imposed on Alexander Bortnikov, director of the Federal Security Service, a successor to the Soviet KGB, and Andrei Yarin, a Kremlin official “whose role was to counter Alexei Navalny’s influence in Russian society including through operations meant to discredit him,” according to the E.U. statement. The others sanctioned are Mr. Putin’s first deputy chief of staff, Sergei Kirienko; a regional boss; two deputy defense ministers; and the Russian chemical weapons research institute that developed Novichok.

Where is the United States? President Trump’s dubious affinity for Mr. Putin should not inhibit U.S. sanctions in concert with Europe. Mr. Putin undoubtedly expects to ignore the fuss and do nothing, as was the case in previous attacks and murders. He needs to hear — loud and clear — that attempting to kill your critics is wrong and has significant costs.

Read more: The Post’s View: It’s no wonder Putin thinks he can get away with poisoning his adversaries Jackson Diehl: Putin is an expert at poisoning dissidents. Trump shouldn’t just let it happen. Edward Fishman, James Lamond and Max Bergmann: No, Trump has not been ‘tough’ on Russia The Post’s View: Putin is running a disinformation campaign on Navalny’s poisoning Vladimir Kara-Murza: The Russian opposition wins at the polls — while their leader recovers from poisoning

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