Russian President Vladimir Putin. Photo: YURI GRIPAS/UPI/PA Images

The attempted poisoning of a former Russian double agent in Salisbury has broken all the rules. Not just those on criminal violence, or on acts of aggression in foreign countries. By attempting to assassinate Sergei Skripal, Vladimir Putin has fundamentally changed the rules of the spying game. The implications of that are far-reaching.

In 2004, Skripal was caught by the FSB and jailed in Russia for passing secrets to MI6. But in 2010, he was pardoned by the then Russian President Dmitry Medvedev, and exchanged along with three other former spies in a handover at Vienna airport. When it comes to former agents who have been exchanged in this way, the unwritten rule of the Cold War was well-known—once handed over, they were considered out of the game. They were no longer targets, and neither were their families. That’s how it went.

Until Skripal, that is. He breaks that mould. Putin has now made clear that Russia will attack its enemies wherever they are and even if that brings risks for the Kremlin in the form of sanctions and international isolation. Coverage of the attack in the Russian state media has been of denial—but in parallel with statements that this is what happens to traitors. That mirrors Putin’s own position.

The old convention is gone. Putin’s message to those who’ve acted against Russia is clear—there are no rules.

That leaves us facing a wall of questions. Here are five:

Why has this happened?

It is unlikely that the attack was directly related to the Russian presidential election. After all, the result of that vote is not in any doubt. Putin will win by a huge margin—he doesn’t need to drum up support.

The motivation behind the attack is suggested by its very outlandishness. In the cold calculation of these things, there are straightforward ways to kill someone, and there are the more complicated methods. Leon Trotsky, the first in the long line of overseas Russian assassinations, was dispatched in 1940 using an ice axe—a cumbersome, but direct method. The use of a highly toxic nerve agent in broad daylight on foreign soil, as was the case with Skripal, is about as baroque an assassination method as it is possible to dream up.

The eye-catching method is a warning, directed at the…

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