We also know that the Kremlin has done this before. In 2009, Russian lawyer Sergei Magnitsky, who had uncovered a massive fraud perpetrated by a group of senior government officials, died in prison following gross mistreatment like that Mr. Navalny is enduring. The Western businessman who employed him, William Browder, has spent the past decade seeking justice in the case, including by inducing the United States and other Western governments to enact laws providing for the sanction of all those involved in Mr. Magnitsky’s case as well as in other human rights violations. Mr. Browder calls what is happening to Mr. Navalny a “slow-motion assassination”; Amnesty International says the regime is creating “a situation of a slow death and seeking to hide what is happening.”... The United States and other Western governments have taken some steps to support Mr. Navalny, including sanctions against officials and entities involved in his poisoning and imprisonment. But if his life is to be saved, much stronger action is needed. Mr. Browder argues that the right targets are the 35 oligarchs whom Mr. Navalny himself has identified as the holders and protectors of Mr. Putin’s massive private fortune. Start freezing assets and applying visa bans to those tycoons and their families, he advises, and keep going until Mr. Navalny is released. That sounds like a strategy that has a chance of working; if it doesn’t, it will be a head start on what must be the consequences if the Kremlin’s creeping murder plot goes forward.