London says Russians used a nerve agent on British soil. How should Britain respond?

Add a new word to your vocabulary: Novichok. It's a chemical weapon developed in the Soviet Union in the late 1980s, a nerve agent reportedly 10 times more potent than its better-known predecessors. A Russian scientist who was accidentally exposed to a small amount reported seeing "brilliant colors and hallucinations"; he died about five years later.

Larger doses may lead to immediate paralysis of the entire nervous system; that's what the British government now believes happened to Sergei Skripal, a Russian defense intelligence officer who spied for Britain, as well as his daughter, Yulia, when they were exposed to Novichok in Salisbury, a provincial English market town best known for its lovely cathedral.

The contrast between the sinister Russian poison and the middle-class, middle-England backdrop of Salisbury - also home to a well-preserved copy of Magna Carta, the foundational document of the British legal system - is part of what has made the story so sensational in the United Kingdom.

Vladimir Putin, a man whom most Britons know as a semi-fictional bad guy who sometimes appears on the evening news, has suddenly insinuated himself into ordinary life. One British policeman fell gravely ill after being exposed to the Skripals; 12 others were hospitalized; hundreds of others have been warned as well. That makes it difficult to dismiss this story - "Russians killing Russians, and why should we care?" - as many Britons did when Alexander Litvinenko, another Russian ex-spy, was murdered with radioactive polonium, another rare, highly classified poison, in central London in 2006.

But while Litvinenko's assassins might have expected to go undetected, the Skripals' would-be murderers had to know that Novichok would quickly be linked to the Russian government. On many levels, this assassination attempt represents whole new levels of defiance. It broke the rules of spy swaps: Skripal had been pardoned by then-Russian President Dmitry Medvedev in 2010 and traded for a bevy of Russian spies.

It showed no concern for bystanders. It follows a long series of other mysterious deaths, including that of Nikolai Glushkov, a Putin opponent, in London on Monday. It showed no concern for bystanders. It has been accompanied by a loud and arrogant disinformation campaign. Since the story broke, Russian state television has alternately issued warnings to "traitors" and blamed British secret services, Georgia and Ukraine.

A Russian politician appeared on the BBC ranting about a new Reichstag fire and comparing the British prime minister, Theresa May, to Hitler. The Russian Foreign Ministry and the Russian Embassy have been openly mocking Britain, tweeting photographs of James Bond and laughing at the evidence.

But why are they doing this? Speaking to British politicians and officials over the past week, I've heard a range of explanations. Just like the attack on the journalist Anna Politkovskaya more than a decade ago, the hit may have been meant as a warning to other potential double agents: You don't have to murder every journalist, or every spy, to frighten the rest.

Alternatively, it may have been designed, in line with the old Soviet tradition of "active measures," to provoke an angry response: In advance of the preordained Russian election next week, Putin can increase sluggish turnout by shouting about "Russophobia" in the United Kingdom.

More ominously, it may have been designed to expose Britain's new isolation: Now that it is leaving the European Union, the United Kingdom no longer has a set of allies it can rely upon to help craft a response.

It has no favors it can draw upon either: For the past year, British diplomacy has been focused on Brexit to the exclusion of all else. As if to underline this weakness, even the White House was stunningly opaque, condemning the attack but repeatedly refusing to mention Russia. The American president, so quick to insult Meryl Streep and Alec Baldwin, has yet to tweet a single syllable.

So extraordinary does this failure seem to the British - does the American president believe their government, or the Russian Embassy? - that here in London, many are asking whether Trump fired Rex Tillerson because the State Department's statement on the Skripal poisonings used the word "Russia," and offered more support than the president himself.

If the point was to expose British isolation, it has succeeded: There is no obvious, fast response that the United Kingdom can make, by itself, that really damages Putin. Bar some more senior Russian politicians from Britain? That hardly matters to the Russian president. Boycott the soccer World Cup? No one will mind if a few members of the royal family don't show up, or if the English team drops out.

The responses that might really matter are much more difficult. The British government could initiate a cyberattack or reveal some hacked information, and there are rumors that it will. It could reinforce its troops on the Russian border, in the Baltic states. It could also decide on much more revolutionary financial actions, make full use of its own new laws on "unexplained wealth" and begin immediate investigations into properties such as 4 Whitehall Court, where the Russian first deputy prime minister is believed to own two apartments worth a total of $15 million, and freezing the assets of any Russian officials in Britain. It could pass new laws making it difficult or impossible for anonymous shell companies to own British property or indeed to function in the United Kingdom at all.

That would cut down on Russian money-laundering, and indeed international money-laundering, and would slowly drive the Russian oligarchs out of London. But it would also cut down on the profits of the real estate agents, yacht salesmen, couturiers, lawyers and accountants who make their living off the international rich who have found the city so welcoming. For two decades, successive British governments put profits over security and underestimated the danger of hosting unscrupulous kleptocrats in their country. The appearance of Novichok in a quiet English town exposes the risks of that policy - just at a moment when Brexit Britain will find it most painful to abandon.

Anne Applebaum is a Washington Post columnist.She is also the Director of the Global Transitions Program at the Legatum Institute in London.