WASHINGTON – Russia’s alleged use of a chemical weapon on British soil is a sign that the United Kingdom and its allies must rethink what deterrence looks like in the 21st century, two senior British defense officials said Tuesday.

Stephen Lovegrove, permanent secretary of the Ministry of Defence, called the attack a criminal act of “attempted murder” and one that requires such a response to deter leaders in Moscow from further attempts.

“It’s not spy on spy. This is an egregious breach of international law and the chemical weapons convention, [an] unlawful use of force by the Russian federation on British soil against a British citizen,” Lovegrove said.

But the bigger issue may be that the attack was “merely a grotesque and outrageous continuation of the type of behavior that Russia has been conducting against any number of countries,” whether through cyber assaults or propaganda, he added.

As a result, the traditional version of deterrence needs to be rethought, said Gen. Gordon Messenger, vice chief of the Defence Staff, who appeared with Lovegrove at an event hosted by the Defense Writers Group.

The situation “does raise quite profound questions about our posture, force, attribution, deniability, which range much further than merely tit for tat spy expulsions,” Messenger said. “Deterrence needs to play into the here and now; and that we need to develop techniques and processes which deter and counter this kind of activity on a steady state basis.”

On March 4, a former double-agent named Sergei Skripal and his daughter were found poisoned in Salisbury, England. The weapon used against them has been identified as Novichok, a chemical agent developed under the Soviet Union; in response, the U.K. and its NATO allies have expelled a number of Russian officials from their respective countries.

Although this appears to be a coordinated use of a chemical weapon by Russia on NATO soil, the U.K. has not invoked Article 5, which requires NATO members to come to the country’s aid. And the reason appears to tie back into the question of what deterrence means in an era of so-called gray zone warfare.

Speaking exclusively to Defense News after the event, Messenger noted that Article 5 was developed for an era when conventional military incursions were the only true concern for the alliance, a clear-cut issue that would trigger the NATO nations to come to each other’s aid.

“The nature of what we face now complicates that. There’s absolutely no question about that,” Messenger said. “What it means is the response of the international community needs to be nuanced and tailored and judged in a very different type of strategic environment than the one in which the conventional incursions was seen as the greatest threat.”

“I think Article 5, as a manifestation of collective security and solidarity, is absolutely fundamental. But that’s not to say more nuanced judgement and other things need to be” considered as well, he said.

Given that, Lovegrove still noted that Article 5 remains a vital standard for reassurance among allies.

“I pick up no sense from any of our NATO partners that the integrity of Article 5, however, is even remotely in doubt,” he said. “That’s something that everybody takes a great deal of comfort from.”