George Orwell recounts in Homage to Catalonia the moment he heard the crack of gunfire on Las Ramblas in Barcelona. When the music stopped, the crowd ran to take shelter in the Metro station but he decided not to follow because he might be trapped for hours.
It's a choice that is now a familiar one to anyone on what should be an unthinking excursion on European streets. The scenario that unfolded in Barcelona on Thursday had already been seen in Britain, France and Germany. The van mowed down people regardless of nationality. Shopkeepers became reluctant heroes for sheltering the targeted. The perpetrators that sought death were stopped by an astonishingly rapid police response.
In the aftermath of the attacks, the well-visited site stood silent behind police tape. Soon the carnival atmosphere will return as visitors come back to build memories.
The crowds quickly returned this summer at London Bridge and the nearby Borough market. And to the Nice promenade that was attacked last summer. The Christmas market that was attacked in Berlin last year will open again in the next few months.
Yet every atrocity has its own context and consequent dangers. Spain has not had a terror attack rooted in Islamist extremism since 2004. That bomb attack on the commuter train in suburban Madrid was instructed by Al Qaeda, which claimed to be extracting revenge for Spanish involvement in the Iraq war.
Political circumstances mattered hugely in the aftermath of that attack. Jose Maria Aznar, then the prime minister, was campaigning for re-election. The ruling party, including the government spokesman and today's prime minister Mariano Rajoy, was desperate to assign blame to ETA the Basque separatists.
The voters knew the government was feeding them a falsehood and revolted. The government lost the election. In effect, Al Qaeda changed the course of an election in Europe. Spain soon pulled its troops from southern Iraq.
READ MORE Barcelona terror attack: What we know
The potential for Thursday's attack to have far reaching consequences for Spain is again all too real. As it was in Orwell's 1930s, Barcelona is at loggerheads with Madrid. Catalonian authorities are pressing ahead with plans to hold a referendum in October on independence.
Mr Rajoy has fought fiercely against the very idea of the vote. So far the fight has been in the courts. Even so Catalonian civil servants have faced fines and jail for their work on the plebiscite.
A terror attack now raises the stakes for the politicians. Clashes over the poll will now be seen against an altered backdrop. The lessons of 2004, particularly Mr Rajoy’s clumsy role, stalk the present.
There has already been a sense that Barcelona faces its own particular terror challenges. Although Spain has been spared Islamist atrocities there was a close miss in the city centre in 2008 when three Pakistani men were captured plotting an attack.
More from Opinion
Prof Peter Neumann of the International Centre for the Study of Radicalisation at King's College London has been working with Catalonian police on counteracting extremism since that time. To Prof Neumann, the attack on Las Ramblas was inevitable. Barcelona is a hub of radicalisation on the north shore of the Mediterranean.
Radical Islamists moving between north Europe and North Africa have put down roots there. The pressures on the security forces to penetrate these groups and neutralise the recruiters has grown steadily.
A study of those arrested for terror crimes in Spain from 2013 until 2015 for the US Military Academy's Combating Terrorism Centre (CTC) found that almost a quarter came from Barcelona. This new generation has been radicalised since the start of the Syrian conflict and the rise of ISIL.
It found that two factors were common in the vast majority of cases. The first, most obvious, was contact with a radicalising agent who convinced the suspect to embrace violence.
The second was social ties with ISIL loyalists or other extremists. An astonishing 87 per cent were radicalised in the company of others. Established social or family ties with a person either convicted of terror offences or who had travelled to Syria or other foreign conflict zones featured in 69 per cent of arrests. The melding of online and real world extremism in Spanish terror is a toxic reality.
Social interactions are a particularly thorny challenge for Spain. North Africans, particularly Moroccans, are a large part of the Spanish population. As the daily headlines this month have made clear, migrants clamour to move north from the African coast in ever greater numbers. Yet the country is a widely acknowledged laggard in integrating its communities. Divisions such as those between the Catalan speakers and Spanish speakers have existed for centuries.
Fernando Reinares, the author of the CTC report, points out identifying and tackling those promoting and carrying out radicalisation is a task for the security forces.
Breaking open the interwoven knot of personal ties and community links within networks that justify terrorism is Spain’s larger unifying mission.