The U.K. announced today that it would expel 23 Russian diplomats and cut off high-level diplomatic contact with Russia, in response to the attempted assassination in London last week of Sergei Skripal, a former Russian intelligence agent.

Skripal was poisoned with Novichok, a dangerous nerve agent known to have been produced in Russia, which also infected his daughter. Individuals near the scene, some 500 in all, were advised to disinfect their belongings.

Why it matters: This attack is the latest in a string of Russia-linked assassinations on British soil, which include the 2006 polonium poisoning of Alexander Litvinenko, also a former Russian intelligence officer, and 14 other Russian nationals who died under mysterious circumstances, such as the oligarch Boris Berezovsky. Yet the Skripal case stands out as particularly egregious and escalatory.

According to the British government’s assessment, the Russians released a chemical weapon on British soil (unlike polonium, Novichok was manufactured specifically as a bio-weapon). Moreover, the Kremlin’s response to Prime Minister Theresa May’s Tuesday night deadline to provide an explanation included denial and a lightly veiled nuclear threat — a reminder of President Putin’s warmongering speech flaunting Russia’s new nuclear capabilities.

After Russia blew that deadline, May reportedly discussed with the National Security Council a wide array of countermeasures, escalating from diplomatic expulsions to sanctions on Russian companies, a crackdown on illicit Russian assets in the UK, a ban on the operations of RT (Russia’s state-owned propaganda arm) and even offensive cyber operations against Russian targets. Leaders of the U.S., Germany and France all offered support for such responses the U.K.

The bottom line: In choosing mainly to expel Russian diplomats, the U.K. took the weakest option. May missed an opportunity to show united transatlantic resolve in the face of escalatory Russian aggression. The Kremlin has learned another lesson from the stand-off: Intimidation works.

Alina Polyakova is the David M. Rubenstein Fellow for Foreign Policy at the Brookings Institution.