Mr. Putin has faced little backlash for actions even bolder than the gruesome intrigues in Britain, like the attacks by Russian forces in Ukraine and Syria. With growing support from autocratic forces in Europe, he must not be emboldened to think he will be unchecked. While President Trump has allowed Mr. Putin a free hand to meddle in American politics, he cannot ignore yet another attempted murder of a Putin foe on allied soil. The administration needs to enforce sanctions Congress has already passed and press NATO to do more, perhaps banning travel by Putin cronies and enacting other restrictions on business activities.

But while Sarah Huckabee Sanders, the White House press secretary, said the administration was “standing with our U.K. ally,” she would not say whether it thought Russia was responsible for the attack.

The Russians knew that the British would identify the nerve agent, leaving no doubt who was behind the killing. That makes Mr. Putin’s typically cynical treatment of the killing doubly outrageous. When the British “sort out” the killing, he said nonchalantly, then he will comment on it. At the same time his top propagandist on state television, Dmitry Kiselyov, was feeding the outlandish story that Britain poisoned Mr. Skripal to create a pretext for boycotting the 2018 World Cup tournament in Russia. Why would Russia bother to go after a double agent of no use to either side, Mr. Kiselyov sarcastically wondered?

Not to silence him, presumably, because the Russians themselves had released him to Britain in 2010 in a swap for a network of sleeper agents rounded up in the United States. The likely answer was provided by Mr. Putin himself a few months after Mr. Skripal was traded to the West. Asked during his annual give-and-take with reporters in 2010 how he would treat treason, Mr. Putin, a former K.G.B. agent, replied: “Traitors will kick the bucket, trust me. These people betrayed their friends, their brothers in arms. Whatever they got in exchange for it, those 30 pieces of silver they were given, they will choke on them.”

Mrs. May demanded that Russia immediately provide complete disclosure of the Novichok gas program to the Organization on the Prohibition of Chemical Weapons. Then she said the government was drawing up a full range of retaliatory options.