Truly, human beings can get used to anything. Alexander Litvinenko, a former Russian spy who became a naturalized British citizen, contracts a horrible lingering illness after taking tea in a London hotel with three former KGB agents.

Just before Litvinenko’s death on Nov. 23, 2006, doctors identify the cause of his sickness: He has been poisoned by a deadly radioactive material, polonium 210. Traces of polonium are found on the dying man’s teacup, on the aircraft the KGB men took to London and in a number of other locations through which they passed, including their hotel rooms. Almost all the world’s supply of polonium is manufactured in one Russian nuclear reactor. This looks a lot like murder. But murder by whom?

On his deathbed, Litvinenko accused Vladimir Putin of responsibility for his death.

Russian officials have scornfully rejected the accusation. They fix the blame for the death of this one Putin enemy on other Putin enemies: the exiled businessman Boris Berezovsky, perhaps, or else former executives of the confiscated Yukos Oil Company.

The British government has requested the extradition of the man who organized the fatal tea party in London, former KGB agent turned multimillionaire businessman Andrei Lugovoi. The Russians have refused. They say: Give your information to us, and we’ll file charges if we think warranted.

The British were not impressed with this offer. Over the past six years, Vladimir Putin’s political opponents have developed a nasty habit of meeting violent deaths. In no case have charges ever been filed.

(In one semi-exception to the rule, charges were filed against the alleged killers of American journalist Paul Klebnikov — they were promptly acquitted.)

As for sharing information with the Russians, that does not appeal to British authorities either. It’s widely thought that Litvinenko’s killers chose polonium as their weapon because they did not realize how sensitive Western tracing technology has become. They assumed that the minute quantities of polonium necessary to kill would be too tiny to detect. Now the Russians are demanding that the British detail the techniques they used to track the polonium back to Russia. Um, sure.

The standoff over extradition has escalated. On July 16, the British expelled four Russian “diplomats” (read: intelligence officers) to protest the Russian refusal. On July 19, the Russians retaliated by expelling four British officials. What happens now?

The Russians are urging the British to let bygones be bygones. The day after the Russian expulsions, Russian Foreign Minister Sergei Lavrov condescendingly explained that the troubles between Russia and the United Kingdom should be attributed to the “inexperience” of prime minister Gordon Brown. Back in Moscow, Russian President Vladimir Putin expressed hopes that the U.K.-Russian relationship would soon improve. And there is no shortage of Westerners ready to second the Russian view. International Herald Tribune columnist Serge Schmemann, for example, just this week urged Western leaders to “discard … Cold War stereotypes and start treating Russia as a potentially constructive partner.”

Well, there’s a thought. Russia as a constructive partner — if only we can just somehow disregard the evidence that the Russian government murdered a British citizen in the British capital with a radiological weapon.

That might seem a heavy challenge. And yet it is a challenge to which Western governments seem more than equal, and perhaps understandably so. If the Russian government really did do what it seems to have done, then the world faces a very grave and frightening problem. This is not Syria we are talking about, some dusty Middle Eastern pariah state. It is a member of the G-8, a veto-wielder at the Security Council, a leading supplier of oil and gas … and on the evidence of the Litvinenko case, it is also a Mafia regime that uses murder as a political tool — and will recklessly expose untold numbers of Londoners to deadly radiation in the process.

If we acknowledged that terrible reality, we would have to do something about it. Since we have no idea what to do, we refuse to acknowledge reality. Worse: We chide those who do acknowledge Russia as dangerous provocateurs and Russophobes, trapped in Cold War stereotypes. But it is not a “stereotype” that Russian government probably ordered the killing of Alexander Litvinenko. Nor is a “stereotype” that Russia is shielding the probable killers. Those are conclusions based on the weight of evidence.

But evidence is the very last thing that most of us want to confront these days, whether about Russia or about Iran or about Islamic terrorism. We are sunk in a mood of denial, in which the only accusations that can get a hearing are accusations against George W. Bush and the United States of America.

The denialists call themselves realists. It’s a strange kind of “realism” that bases itself on fantasies.