Brussels attacks: Terrorism could break the EU and lead to Brexit
Another explosion shatters the morning commute of a major European city; the railway concourses of Antwerp and Paris echo to the clatter of army boots; the borders of the Netherlands, Belgium and France slam shut once more in the hope of stopping terrorists.
While all immediate thoughts go out to the people of Brussels today, the reality is that this latest terror outrage delivers a triple blow to Europe and its cherished notions of open borders, free movement, tolerance and free speech.
Political leaders will call for courage from the public to resist turning inwards, to refuse to allow the terrorists to divide and rule us, but the failure yet again to prevent a major attack makes such calls ever harder to heed.
Firstly, the sight of European borders closing again puts another dent in Schengen, the no-borders agreement that facilitates European cross-border trade and travel, but – as the public well understands after the Paris attacks – the free movement of terrorists too.
Just four days after the capture of the main Paris attacker Salah Abdeslam – a belated victory, as it seemed, for Franco-Belgian security – the jihadist Hydra sprouts another head, commits another atrocity.
No one underestimates the difficulty of stopping random terror attacks on the open street from committed suicide attackers, but these outrages took place at an airport and a metro station - controlled spaces where travellers should expect to feel safe.
Secondly, if – as happened after Paris – it emerges that those responsible for these Brussels attacks were among the migrants who were "waved through" into Europe in the last 12 months, attitudes to migration and multi-culturalism risk hardening still further.
More immediately, while these attacks are likely to drive up public support for the EU-Turkey deal to deport migrants from Greece back to Turkey, they will also undermine a key component of that agreement.
Namely, that Turkey agreed to take back the migrants in exchange for visa-free travel to Europe for Turkish tourists and businessmen. France and Spain were already openly dubious about the wisdom of this quid pro quo; these attacks are likely to render the idea still-born, both practically and politically.
The EU-Turkey deal was always a long shot, but given the closure of the Balkan Route had already choked off numbers, there was a faint chance that a display of determination to deport migrants back to Turkey might have sent the message to the refugees not to bother coming.
With these attacks, the chances of success for the deal become more remote, building further pressure on Europe and Greece over migration at a time when Europe, for the sake of its credibility and unity, desperately needs a deal – however legally dubious and ugly – to stick.
Lastly, looking still further towards the dark horizon, these attacks are likely to sow yet more seeds of doubt in the minds of British voters on June 23.
Downing Street had hoped to make security a key plank of the campaign to remain in the EU, but on days like today, the very phrase “European security” sounds like a bad joke.
Mr Cameron wants to argue that we need to remain part of European agencies like Europol and Eurojust precisely to prevent terrorist atrocities in Britain, but in the rough and tumble of the campaign that counter-intuitive argument will be hard to make.
Inevitably, attacks such as this – striking at the very capital of the European project – risks deepening the urge among some British voters to retreat behind our borders, throwing up the “Brexit” sign as we go.
Mr Cameron and other leaders will argue that would be a short-sighted mistake – unravelling the same Europe now under terrorist assault – but every new attack renders the argument that the EU makes us safer a little harder to make.