British voters’ choice to leave the European Union is unfortunate, but unsurprising. For decades, British politicians have avoided making the case for EU membership, or even explaining to the British people how the Union works and why it is necessary.

Throughout his premiership, David Cameron failed to show any leadership or willingness to engage in a meaningful way with the EU. One foot was always out the door as he repeatedly railed against faceless bureaucrats in Brussels. Sadly, his last-minute attempt to defend EU membership during the referendum was not enough to undo the effects on British public opinion of decades of lies.

Britain has chosen isolation in Europe, against the advice of its friends and allies. Now that its post-imperial decline is complete, the most important lesson to take away from the EU referendum that sealed its fate is that one cannot defeat nationalism by pandering to nationalists. If the EU wants to tackle the strains of nationalism that are undermining its very raison d’etre, it will have to listen to people’s concerns and offer a radical new vision for effective governance. Otherwise, the nationalist cancer will spread.

To begin with, the remaining EU countries must now uphold European law and push for a quick and clean divorce. The United Kingdom’s citizens voted to leave, and its political leaders insist that they will respect the referendum’s outcome, so it is not “punishment” to insist that they do so as soon as possible. If British inaction causes Europe to continue to suffer economically from political uncertainty, a full separation should be carried out unilaterally.

Politically, the UK is already on its way to becoming an adversary, rather than a trusted partner, of the EU. Before reversing herself under fire, Theresa May – the home secretary, who is now the leading candidate to succeed Cameron as Prime Minister – implicitly threatened the future status of EU nationals living in the UK by promising only that they would be “part of the negotiation” for Britain’s exit.

May actually opposed Brexit. Yet her anti-European hostility differs only in degree, not in kind, from that of pro-Brexit politicians, such as Daniel Hannan, a Conservative MEP, and Nigel Farage, the leader of the UK Independence Party, who rejoice at the possibility of additional exit referendums across the EU.

Fortunately, post-Brexit contagion has so far failed to materialise, most likely thanks to the embarrassing public spectacle that the UK has become since June 23, with its economic prospects dimming, its financial sector scouting new locations, and its leaders stabbing one another in the back. In fact, the referendum may have even prompted EU countries — including those with noisy Eurosceptic parties, such as Denmark and Sweden – to close ranks in support of EU membership. A post-referendum poll in Sweden found 66 per cent of respondents still in favour of EU membership. In Denmark, similar polling after the Brexit referendum showed a 9 per cent jump in support of EU membership.

EU leaders must not sit on their hands. The Brexit crisis should be treated as an opportunity for the Union, given that concerns about globalisation, terrorism, immigration, and inequality are now mainstream. A divided Europe has failed to respond to these challenges; and, while the British will be missed, the EU will now be less divided.

Still, today’s EU is not capable of decisive, large-scale action. To provide an alternative to the emotional appeal of populist nationalism, the Union must become more responsive to its citizens’ demands. This will require radical structural reforms to the Eurozone and to the EU’s central political institutions.

Economists generally agree that a single currency without a fiscal union or a common treasury is unsustainable. If Europeans do not finish the job they started when they established the euro, they will continue to experience the economic pain of structural disunity. Complete integration would deliver prosperity and better governance.

Europe’s security arrangements suffer similar shortfalls. It is absurd that Europe has the globally respected European Space Agency, but no credible organisation to collect and share intelligence in the fight against terrorism. The attacks in Paris last November should have made it obvious that borderless terrorism demands borderless intelligence, yet EU member states continue to put individual sovereignty ahead of collective security. More inter-governmental cooperation simply won’t do. Europe needs a central intelligence authority with teeth.

Only by starting on such far-reaching reforms can Europe begin to stem the rise of populism that led to the success of the UK’s “Leave” campaign. Some would say that more sovereign powers should be returned to EU member states. I could not disagree more. This would only further weaken the project of integration that has safeguarded peace and prosperity in Europe for decades. Populism and Euroscepticism are the enemies of that project. The only way to beat them is to build a Europe that works for its citizens. Anything less works only for the demagogues who would follow the UK into the wilderness.

— Project Syndicate, 2016

Guy Verhofstadt, a former Belgian prime minister, is President of the Alliance of Liberals and Democrats for Europe Group (ALDE) in the European Parliament.