Another Putin foe is felled. The West must respond. Russian opposition leader Alexei Navalny delivers a speech during a rally to demand the release of protesters jailed for seeking fair elections, in Moscow in September 2019. (Shamil Zhumatov/Reuters) Opinion by the Editorial Board August 21 at 3:29 am AET IN THE past several years, Alexei Navalny has emerged as the most effective opposition leader ever faced by Russian ruler Vladimir Putin. His Internet videos documenting the corruption of Russia’s rulers, including former prime minister Dmitry Medvedev, have received tens of millions of views. Tens of thousands have turned out for demonstrations he has promoted in Moscow against corruption and election fraud. Recently, he set up a national network of offices to promote challengers to candidates of Mr. Putin’s ruling party in local elections. Mr. Putin, for his part, is struggling with multiple problems, including poor management of the covid-19 epidemic, low poll ratings and the eruption of mass protests in the eastern city of Khabarovsk. So the news that Mr. Navalny was hospitalized Thursday and gravely ill after a suspected poisoning was shocking — but not a surprise. Doctors in the Siberian city of Omsk, where Mr. Navalny reportedly was in a coma and on a ventilator after falling ill on a flight, have not confirmed the charge of poisoning disseminated by his spokeswoman, who said he took ill after drinking tea in an airport cafe. Nor is it certain that an attack on him would have been ordered by Mr. Putin; thanks to his anti-corruption exposés, the 44-year-old activist has other enemies. Still, poisoning has become a common fate of Putin enemies. Other victims have included two former Russian spies exiled in Britain; a member of the protest group Pussy Riot; a former Ukrainian president; and Post contributor Vladimir Kara-Murza, a journalist and opposition activist who has survived two poisonings in Moscow. Boris Nemtsov, the most prominent opposition leader before Mr. Navalny, was gunned down on a bridge near the Kremlin in 2015. Despite the brazenness of these attacks, Mr. Putin has suffered little blowback, other than the expulsion of a few diplomats from Western capitals. Even when, as in the fatal poisoning of Alexander Litvinenko in London, the perpetrators have been positively identified, they have escaped punishment; Mr. Litvinenko’s alleged killer, a former agent of the FSB spy agency, now sits in the Russian parliament. One reason for the impunity is the indifference of President Trump, who when asked in 2017 about Mr. Putin’s record as a “killer,” responded, “There are a lot of killers. You think our country’s so innocent?” Mr. Trump, who benefited from Russian intervention in the 2016 election, has never publicly criticized Mr. Putin; though he has spoken to the Russian leader on multiple occasions in recent months, Mr. Trump has, by his own account, never raised U.S. intelligence reports of Russian payments to the Afghan Taliban for the killing of U.S. soldiers. Mr. Navalny’s family and regular doctor were seeking his transfer Thursday to a foreign hospital for treatment. European governments should offer assistance — and they should let the Kremlin know that if Mr. Putin’s leading opponent was indeed poisoned, there will be, for once, consequences.