Historians may one day be able to describe how, in March 2018, a major tipping point was reached in Europe’s fraught relationship with Vladimir Putin’s Russia. They will also surely ponder how a military-grade nerve agent called novichok, first developed in the Soviet Union of the 1970s and 80s, became a deciding factor in a continent finally pushing back against Russian subversion.

On Monday and Tuesday, 17 EU member states, joined by Norway, Ukraine, Macedonia and Albania, announced they would expel a total of 50 alleged Russian spies in a coordinated response to the chemical weapon poisoning of Sergei Skripal and his daughter Yulia on 4 March in Salisbury. The unprecedented diplomatic move came 12 days after Britain said it would kick out 23 Russian intelligence officers working in the London embassy. The US, Canada and Australia also acted, contributing to the largest ever collective expulsion of Russian diplomats by the west. But this was a seminal moment first and foremost for Europe.

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European reactions are what matters the most to Putin. Russia’s newly re-elected president has long made plain his ambition to overturn Europe’s post-cold war order by upending some of the rules that bind it. The EU has become anathema to Putin for many reasons, not least because it embodies a set of norms that run counter to the very nature of his regime, and because as an economic bloc it can in theory exert leverage on Russia, as none of its other international interlocutors can.

This was indeed a decisive moment for Europe, and not just because of the solidarity demonstrated towards Britain at a time when its relations with the rest of the continent have been damaged by the tedious task of Brexit. Just one year ahead of the EU’s first ever breakup – with the UK scheduled to pull out of the club on 29 March 2019 – European unity was put on display, and very intentionally so.

Facebook Twitter Pinterest Russian diplomats and family members leave the Russian embassy in central London, 20 March 2018. Photograph: Andy Rain/EPA

The most important reason this came about had less to do with Theresa May’s diplomatic manoeuvring than it did with the exact nature and scale of the security threat that has suddenly been identified by Europeans. The first ever recorded use of novichok in Europe, where the use of chemical substances evokes 20th-century horrors, signalled that an entirely new danger had appeared on the continent – one that warranted an extraordinary response.

The key point is that no European government has expressed any doubt about Britain’s assessment that Russia was responsible

Unsurprisingly Monday’s expulsions did not involve all EU states; Greece and Austria were conspicuously absent. But the nature of the support – which encompasses countries that are either candidates for membership or strategically interested in rapprochement with the EU (Ukraine) – arguably said more about European convergence that it did about divergence. Interestingly, Hungary, a country whose leader Viktor Orbán has a notorious track record of pandering to Putin, immediately joined the fray. Nor are these announcements the end of the story: European officials say more measures are yet to come.

The key point is that no European government has expressed any doubt about Britain’s assessment that Russia was responsible. As such, this was nothing like the 2003 debate over the presence, or not, of weapons of mass destruction in Iraq. That involved significant dissent: France and Germany never took the bait.

So how was European unity achieved on novichok and Russia? In London, several politicians and commentators applauded the government for its capacity to rally European support despite the looming Brexit – as if the much decried diminishment of Britain’s influence had suddenly been proved wrong. But sources in Brussels and other European capitals offer a different take on events.

Though British intelligence briefings made available to European partners had an undeniable impact, diplomats were struck that Britain initially refrained from reaching out to the EU as a whole – and attributed this to the awkwardness created precisely by Brexit. Instead, British diplomacy worked at a bilateral level, engaging in particular with eastern European and Baltic states whose hostility towards Russia is well known. That piecemeal approach carried more risks than benefits.

Facebook Twitter Pinterest ‘Entire buildings and squares in Salisbury have to be decontaminated.’ Photograph: David Mirzoeff/PA

In the end, three factors leading to a strong common EU statement and the withdrawal of the bloc’s ambassador in Moscow were crucial. First, British weakness came to be seen as a vulnerability that was shared, not something anyone could possibly gain from in dealing with Russia. Moscow had targeted Britain because it was Europe’s frailest part, the country whose isolation could most conveniently be exposed. Anyone doubting this ought to pay attention to the way Russia’s foreign ministry spokeswoman reacted to Monday’s expulsions: this was, said Maria Zakharova, “a conspiracy of anti-Russian solidarity imposed by the British” on EU countries.

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Secondly, Franco-German resolve was decisive, according to European sources. Emmanuel Macron, Angela Merkel and May met ahead of last week’s EU summit, to parade their unity. Later, when all 28 member states’ leaders gathered, Macron pushed vehemently for the strongest possible collective response. France had long seen itself as playing a leading role in denouncing chemical weapons use in Syria by the Assad regime, Russia’s ally, and it made the case that impunity in the face of such acts was intolerable.

But the third and perhaps the principal explanation for Europe’s decisiveness was that member states saw a clear new threshold had been crossed. Unlike the 2006 killing of Alexander Litvinenko in London by Russian agents (who’d dropped polonium in his tea), the use of novichok was seen as an indiscriminate attack on innocent civilians – not just a targeted assassination. May highlighted this fact on Monday, when she confirmed that 130 people were at risk of exposure. Novichok is reportedly the most deadly nerve agent ever produced, five to eight times more lethal than VX. Entire buildings and squares in Salisbury have to be decontaminated.

This put Salisbury into an entirely different category from previous Russia-connected eliminations of dissidents: in a category closer to terrorism. Novichok, to that extent, was a unifier for Europe – a matter of all for one, one for all. And yet the comfort to be drawn from this is limited. After Crimea, war in Ukraine, nuclear sabre-rattling and cyber-attacks, Russia’s behaviour has taken on an entirely new and baffling degree of recklessness – one that can put anyone in danger. And while they pulled together, European officials noted again the absurdity of Brexit, with Britain’s self-inflicted loss of influence and ensuing vulnerability plain for all to see.

• Natalie Nougayrède is a Guardian columnist