ONE question confronting Britain and the European Union is how to maintain foreign-policy and security co-operation after Brexit. Theresa May, Britain’s prime minister, often notes that failure to find agreement would harm the security interests of both sides. On March 22nd that observation found a pointed form of expression at an EU summit in Brussels, following the recent nerve-agent attack on a Russian émigré and his daughter in Salisbury. After some agile British diplomacy the EU’s 28 leaders issued a joint statement declaring that the only plausible explanation for the attack was that Russia was responsible for it.

This language, tougher than Mrs May had dared to hope, clears the way for further collective EU action, including on beefing up defensive instruments against Russian hybrid warfare, later this year. Nor was the European response limited to words. The EU’s ambassador to Russia has been temporarily withdrawn to Brussels, and on March 26th several countries, possibly including France and Germany, are expected to announce that they are following the British example and expelling Russian diplomats. Angela Merkel, Germany’s chancellor, reportedly asked her fellow leaders to state which of them would agree to do so.

Mrs May, whose experience inside the EU over the past 18 months has largely been one of humiliation, can chalk this up as a diplomatic win. EU leaders jealously guard their foreign-policy prerogatives (which is one reason why some of the promises to expel Russian spies may vanish on contact with domestic reality). To obtain tough language and common diplomatic action looked far from inevitable earlier this week. “Russia is challenging the values we share as Europeans,” declared the prime minister as she left the summit on March 23rd. “It is right that we are standing together in defence of those values.” Boyko Borisov, Bulgaria’s prime minister and no Russia hawk, declared that: “Despite Brexit, the UK and EU showed they are one and the same.”

On first glance Mrs May’s success looks like one in the eye to Remainers who suggested that Brexit would leave Britain diplomatically isolated. It is true that on everyday business British influence inside the EU has dwindled almost to nothing, much to the regret of traditional allies like the Dutch. But the summit demonstrated that Britain can still forge alliances and score diplomatic wins when it counts (especially welcome when its American ally looks increasingly unreliable). Contrast this with the experience after 2006, when Tony Blair’s government was disappointed by the lukewarm support it received from its European allies over the Kremlin-sponsored poisoning of Alexander Litvinenko, a Russian-born exile in London.

Since then a lot has happened, including Russia’s incursions into Ukraine and its support for Bashar al-Assad’s atrocities in Syria. But if EU membership does not guarantee solidarity, it certainly helps. The statement of March 22nd was agreed by the 28 leaders only after Mrs May had convened Mrs Merkel and Emmanuel Macron, France’s president, for a brief pre-dinner discussion. The trio issued a firm statement that prepared the ground for the subsequent debate. Over dinner, concerns from countries that tend to be dovish on Russia were overcome in various ways; Greece and Cyprus, for instance, were softened up with the promise of a tough EU stance on Turkish naval incursions and the recent arrest of two Greek soldiers who had strayed across the border. At its best, this is how matters are supposed to proceed inside the EU: leaders thrashing out differences, balancing competing interests and prioritising concerns before arriving at a common line.

But consider whether the same result could beobtained after Brexit. Mrs May would not have been in Brussels to convene impromptu summits with her French and German counterparts, let alone to make her case before all 28 leaders. British diplomats would have had no role in the multitude of decision-making groupings that constitute the inner workings of the EU, and would be unrepresented at ministerial meetings. The informal diplomatic treadmill would have to be worked with particular vigour. The prime minister would have to ask allies to engineer the sorts of diplomatic pirouettes that made statements such as these possible, and cross her fingers that they would deliver.

It should be possible, one way or another, to keep Britain plugged in to the EU’s decision-making machinery after it leaves. Among other things, its European allies want to maintain access to British terrorist databases and intelligence assessments that undergird much of the EU’s sanctions policy. The negotiating guidelines for Britain’s post-Brexit relationship with the EU, signed off by the remaining 27 leaders on the morning of March 23rd, call for “strong EU-UK co-operation” in foreign and security policy. So Britain will not be isolated after Brexit. But its government will find it much harder to conjure support from outside the tent.