British Prime Minister Theresa May. (Will Oliver/EPA-EFE/Shutterstock)

THE USE of a military-grade Russian nerve agent in an attack inside Britain shows that Western governments are failing to deter the regime of Vladi­mir Putin from increasingly audacious acts of aggression. The British government’s conclusion that it is “highly likely” that Russia carried out the attempted assassination of a former spy and his daughter follows by weeks a military assault, backed by artillery and tanks, by Russian irregulars on U.S. positions in Syria. That in turn follows Moscow’s attempts to interfere in multiple Western elections in recent months, and repeated incursions by Russian warplanes into Western airspace.

Mr. Putin keeps doubling down on his provocations — some of which are unprecedented, even by the standards of the Cold War — because he has concluded he will pay no significant price for them. Britain, which shied away from tough action after a previous Russian political assassination on its soil, on Wednesday announced a more robust response, including the expulsion of 23 suspected Russian spies. But it will take more than action in London to stop Mr. Putin. A concerted response by the Western alliance is needed.

It’s worth underlining the magnitude of the offense represented by the attack last week in the normally peaceful city of Salisbury. The chemical agent identified by British scientists, known as Novichok, was secretly developed by the Soviet Union beginning in the 1970s, in violation of Moscow’s subsequent treaty commitment to stop developing and producing chemical weapons. The agent is believed to be 10 times more powerful than other weaponized nerve agents, such as VX. That it would be employed in a civilian setting, endangering not just former spy Sergei Skripal and his daughter but scores of other people, too, is not just a crime but also an act of state terrorism.

The distinctiveness of Novichok means that Mr. Putin must have calculated that Russia would be identified as the author of the attack. Perhaps he wanted it that way: He was head of Russia’s spy agency, the FSB, at the time that Mr. Skripal allegedly supplied information to Britain, and had publicly vowed revenge. Most likely, he was also encouraged by the relatively weak British response to the killing of another former spy, Alexander Litvinenko, in London in 2006. Eight months after that attack, Britain finally expelled four Russian diplomats; but a full inquest, which concluded Mr. Putin was probably responsible for the killing, was not completed for nine years.

Though the new British sanctions package is more forceful, it needs to be backed up by other Western governments, starting with the United States. All too characteristically, President Trump has equivocated about Russian responsibility for the Salisbury attack; and the administration has still not implemented sanctions mandated by Congress in response to past Russian offenses. Congressional leaders should insist that the sanctions go forward. It is imperative that the United States support its closest ally when it has been subject to such extraordinary aggression.

An adequate international response to Mr. Putin would push back against his ventures on all fronts: Syria, where the United Nations has found Russia complicit in war crimes; Ukraine, where Russian-backed forces continue to seek military advantage; cyberspace, where Russian hackers and bots remain ubiquitous. In the absence of such action, Mr. Putin’s ambitions, and his audacity, will only escalate further.