By Shashank Joshi

(CNN) -- The coordinated expulsion of over 100 suspected Russian intelligence officers -- from countries across the European Union, NATO and beyond -- in response to the attempted assassination of former Russian spy Sergei Skripal and his daughter on March 4, is a remarkable diplomatic coup for Britain.

It represents not only the largest American expulsion in more than 30 years, but also the largest collective expulsion in intelligence history.

While the UK might have expected its closest allies to follow its lead, it could not have imagined that non-NATO members, like Finland, non-EU members, like Albania, and countries with strong ties to Russia, such as Hungary, would also take part.

Given President Donald Trump's refusal to criticize Russian President Vladimir Putin, America's mass expulsion also comes as a pleasant surprise.

The UK's success in building this broad coalition is down to a number of factors.

One is the nature of the provocation, which, as the European Union pointed out in its statement, represents the first use of chemical weapons on European soil since the Second World War.

Russia's own role in enabling and then concealing its Syrian ally's own deployment of chemical weapons over the past several years, thus breaking down the norm against their use, will have been high in the minds of Western leaders.

The strong response is also a testament to the work of British investigators, who have been able to present compelling evidence to persuade even their most skeptical partners.

And while the UK has invited the Organisation for the Prohibition of Chemical Weapons to investigate the case, bolstering its credibility, Russia has responded with an astonishing array of blanket denials, conspiracy theories and attempts to blame countries as diverse as Sweden and the Czech Republic.

But the UK also got lucky. It was able to marshal this response not just because of the horror of chemical weapons or its investigating prowess, but also because Russia has tested the West's patience to breaking point.

Germany, announcing its expulsion, cited not only the Skripal attack, but also what it said was a Russian intrusion into the German government's secure data network.

Australia cited a "pattern of recklessness and aggression," including the downing of a Malaysia Airlines flight over Ukraine and interference in Western elections.

Estonia, which expelled Russia's defense attaché, noted in the annual report of its intelligence service last year that the Czech Republic, Germany and Sweden, among others, had noticed an "increased intensity of Russian intelligence operations."

Skripal, then, was the straw that broke the camel's back. Despite four years of European sanctions -- imposed after the illegal invasion and occupation of Ukraine in 2014 -- Russia's aggressive behavior has driven the country further into isolation, right across the West. Unwinding this state of affairs -- lifting sanctions, rejoining the G7 and rebuilding ties with Europe -- will take years.

Russia's isolation is extraordinary, but it is not absolute. While the extent of support for the UK was unprecedented, the holdouts are also notable.

Portugal, Austria, Bulgaria, Cyprus, Malta, Slovakia, Slovenia and Luxembourg have all refused to expel diplomats. This will raise some concerns over Russia's ability to split the European Union by peeling away individual states. Austria's far-right Freedom Party, which controls the interior and defense ministries, has formal ties to Putin's United Russia party.

Bulgaria, where Russian investment makes up a fifth of gross domestic product, is due to host Putin later this year.

Cyprus is a hub of Russian money laundering, and three years ago signed a deal to allow the Russian Navy use of its ports. As the EU considers how to defend itself against Russian hybrid warfare, it will have to think carefully about how to handle these dissenting members, who represent chinks in the bloc's armor.

Finally, will this make a difference?

The purpose of expulsions is twofold: One is to send a message of deterrence, demonstrating that flagrant attacks will not go unpunished and that the Russian state will pay a price. The second, no less important, is to make it tangibly harder for Russia to use its intelligence services to penetrate, subvert and intimidate Western nations.

This is not just about curbing old-fashioned espionage, but a broader challenge.

As one former senior British intelligence official has pointed out writing anonymously for the Royal United Services Institute, where I am a fellow, spies working out of embassies can provide support for operations such as assassinations, as evidenced by Qatar's arrest of a Russian GRU (military intelligence) officer in 2004 after a car bomb attack on a Chechen separatist.

Moreover, coordinated expulsions -- rather than those from one country alone -- are especially valuable, because intelligence agencies often operate on a third-country basis. A Russian spy will find it easier to target and recruit a British national in Cyprus or Malta, where local security services are weaker and more indulgent of Russian activity, than in London. But with more than 20 countries taking part in expulsions, Russian capabilities have been hit across the board, making it harder for Moscow to compensate for losses in any one capital.

It will also be hard for Russia to replenish its rezidentury -- intelligence stations -- because Western partners are likely to share information on suspected Russian intelligence officers more widely than before.

This is the worst week for Russia's intelligence service in decades, and marks the further collapse of Moscow's relationship with the West. The question is whether this blow nudges Russia into a more cautious approach to its adversaries, or whether a re-elected, isolated Putin now lashes out with even less restraint.