Is the Cold War really over? Well into the sixth year of what Norman Podhoretz calls World War IV, the case of Alexander Litvinenko suggests that there is still unfinished business from World War III. The former Russian spy turned critic of the Putin regime appears to have had poison slipped into his tea during a meeting with two mysterious Russian visitors in a London hotel. Mr. Litvinenko is now clinging to life, receiving intensive care under armed guard. His symptoms are consistent with thallium poisoning: His liver is going, his bone marrow is gone, he has lost all his hair and is being fed by tube. A leading toxicologist who is treating him, Dr. John Henry, believes that the thallium may have been radioactive, rendering its effects more lethal but also harder to trace.

Scotland Yard’s counterterrorism department has taken over the investigation, and the Security Service, MI5, is also involved. The Foreign Office has intimated that if a link with Russia’s FSB, successor to the KGB, can be established, there will be consequences.

Mr. Litvinenko was dismissed from the Russian secret service in 1998 with the rank of lieutenant colonel for, he claimed, refusing to obey an order to assassinate Boris Berezovsky, the oligarch who fled to Britain. Mr. Litvinenko was arrested and imprisoned in the notorious Lefortovo prison, where Natan Sharansky and other celebrated dissidents were incarcerated during the Soviet era.

Upon his release, Mr. Litvinenko accused Vladimir Putin, who then was prime minister and now is president of Russia, of justifying the second Chechen war by using the FSB to cause explosions in apartment blocks in Russia, in which some 300 civilians died. His book, “Blowing Up Russia: Terror From Within,” prompted his rearrest, but again the charges were dropped. In 2000 he sought asylum in Britain before a third prosecution could be brought.

London has been a haven for Russian refugees since the tsarist era. The house where Alexander Herzen, the 19th-century writer and thinker considered the father of Russian socialism, wrote his memoir “My Past and Thoughts” still stands in the Bayswater district, where it served as the backdrop for Tom Stoppard’s Broadway hit, “Coast of Utopia.” More recently, London has been home to defectors from communism, such as Vladimir Bukovsky and Oleg Gordievsky.

But London is by no means safe from the machinations of the Kremlin. Back in 1971, no fewer than 150 Soviet spies were expelled. In 1984, the Bulgarian dissident Georgi Markhov was murdered on Waterloo Bridge by a poisoned dart from an umbrella carried by a Soviet agent. Today, the Russian Embassy in Kensington still has an estimated 30 spies on its staff. Their function is mainly industrial espionage, but the use of poison to silence a former intelligence officer living abroad suggests that criminal methods hitherto thought to be restricted to the Soviet Union are now spreading to Western capitals.

The most infamous poisoning of which the FSB is suspected was that of Viktor Yushchenko, the pro-Western president of Ukraine. He survived dioxin, also known as Agent Orange, poisoning in 2004, but his skin was severely damaged. His ravaged face is a permanent reminder of the FSB’s ruthless methods.

But the case of Russia’s most prominent independent journalist, Anna Politkovskaya, is perhaps the most disturbing of all. She was poisoned two years ago on her way to cover the Beslan massacre, and then, only last month, she was brutally gunned down by an assassin who lay in wait for her at her apartment building in Moscow. The silence of the West about her murder has been deafening.

How seriously these cases are taken depends on where we draw the lines that divide diplomacy from espionage and espionage from war.

Diplomacy has rules and must respect the law. Diplomats enjoy privileges, which, if abused, must be withdrawn. So powerful has Russia become as Europe’s main energy-supplier that it no longer fears for its status in places like London, and accepted norms have fallen into desuetude.

Declaring a few Russian diplomats persona non grata does not do justice to the gravity of the case. Once espionage turns to assassination, it ceases to be a tool of diplomacy and becomes an act of war. The classic definition of war by Carl von Clausewitz is: “An act of force to compel our enemy to do our will.” The Kremlin’s assassins are not merely revenants from the past — they are still fighting the Cold War.

After all, President Putin makes no secret of his aim to restore Russia’s “sphere of influence” by whatever means he deems necessary. Both his goals and his methods are dangerous to Western interests.

Under Mr. Putin, the Kremlin has exploited ethnic and religious tensions in Ukraine, Belorussia, the Caucasus, the Baltic states, and the central Asian republics. These countries are too weak to defend themselves. Russia has also supplied nuclear technology to enemies of the West. At home, the Putin regime has fixed elections, stifled opposition parties, and is well on the way to abolishing the independence of the judiciary, the rule of law, the free market, and the free press.

Not a single report about the Litvinenko case by Russian journalists dared remain neutral, let alone point a finger at Mr. Putin. The press meekly regurgitated denials by the Kremlin’s spokesman, Dmitry Peskov: “We cannot comment on the very fact of what happened to Litvinenko. We don’t consider it possible to comment on the statements accusing the Kremlin because it is nothing but sheer nonsense.”

Sheer nonsense? The last person to see Mr. Litvinenko before he fell ill more than three weeks ago warned him of a threat to his life. Their friend Politkovskaya had already been murdered: He would be next. One of the two Russians who met him earlier that day was a former FSB bodyguard named Andrei Lugovoy who has now disappeared. We do not know who the other man was. Both are presumed to be back in Russia and hence outside the rule of law. The evidence is circumstantial, but only an organization like the FSB is capable of assassinations using radioactive thallium. We know enough to say that what happened to Mr. Litvinenko looks very like what happened to other Russians who became — in a much more sinister sense than Russian writer Ivan Goncharov portrayed — “superfluous men.”

The West’s response to the murder of Politkovskaya was to welcome Russia into the World Trade Organization. We forget how important American and British support was to the dissident movement during the communist era and how trade could be used to extract concessions from the Soviets. In the early 1970s, the Jackson-Vanik Amendment linked free trade to the right to emigrate — a crucial victory for Jewish refuseniks in their campaign to go to Israel. The octopus tentacles of the Kremlin are reaching into the West’s own backyard to suffocate the dissidents of our time. If we ignore them, if indeed we fail to trace the evil back to its source, we deprive millions in Russia of that most precious source of freedom and justice: hope.