Two days after the 9/11 attacks in 2001, a 31-year-old called Nizar Trabelsi was detained in Uccle, on the outskirts of Brussels. The former footballer of Tunisian origin was eventually sentenced to 10 years in prison for an intended attack on a Nato base.

In 2003, another militant of Tunisian origin detained in Belgium was sentenced to six years in prison for his role in a Brussels-based network which supplied fraudulent documents that had allowed the killers of an Afghan commander, Ahmed Shah Massoud, who fell victim to a suicide bomber two days before 9/11, to reach their target.

In the first half of the last decade, as European security services struggled to understand the new threat they faced, and bombs exploded in Madrid and London, Belgium was largely ignored.

Given the increasingly evident role that the country has played in the Paris attacks, this now looks like a mistake. By some estimates, Belgium has supplied the highest per capita number of fighters to Syria of any European nation – between 350 and 550, out of a total population of 11 million that includes fewer than half a million Muslims.

This is a problem that has been building for many years. As elsewhere in Europe, Belgium suffered waves of terrorism in the 1980s and 1990s linked to unrest in the Middle East. “There is a very long history of connection between Belgium and France in the realm of terrorism,” saidRik Coolsaet, an expert in terrorism at the University of Ghent.

In the first half of the last decade, a few score men from Belgium made their way to Iraq. So too did Muriel Degauque, a convert from Charleroi, who died in 2005 while bombing a US convoy in Iraq, becoming the first European woman to launch a suicide attack.

Others travelled to Afghanistan. In 2008, a network sending young Belgian Muslims to al-Qaida training camps was broken up. Many appeared disappointed by what they found in the combat zone but that did not seem to slow the flow. Several returned with the intention of committing attacks at home, prosecutors claimed.

The leader of the group was Moez Garsallaoui, a Tunisian-born Belgian based on Pakistan’s border with Afghanistan. Garsallaoui was not part of al-Qaida, but formed his own faction, fighting international forces in Afghanistan and fomenting terror back in Europe. Before being killed by a US drone strike in 2012, he trained Mohammed Merah, the 23-year-old French former petty criminal who killed seven people in Toulouse and Montauban.

The violent aftermath of the Arab spring provided a new motivation and new opportunities for extremists all over Europe, and militancy in Belgium itself was already broadening and deepening. One well-known, ostensibly non-violent, group of activists – Sharia4Belgium – that was responsible for a stream of aggressive hate-speech and alleged recruitment was eventually broken up.

Molenbeek, a borough of 90,000 in the capital where some neighbourhoods are up to 80% Muslim, is seen by many as a particular problem. “In parts of Brussels, there are areas on which the police have little grip, very segregated areas that don’t feel they’re a part of the Belgian state,” said Edwin Bakker, a Dutch analyst.

Others say the role of Molenbeek is being exaggerated and that self-help groups, especially those run by concerned parents, have made a real difference there in recent years.

But the two trends – the flow of young disaffected Muslims to Syria and the longstanding links to France – inevitably combined. In August 2014, a Frenchman of Algerian origin living in Molenbeek shot dead four people at the Jewish museum in Brussels. Mehdi Nemmouche had recently returned from Syria, French officials believe, where he may have acted as a jailer of western hostages for Islamic State.

When Amedy Coulibaly, the Frenchman who claimed allegiance to Isis when he attacked a Jewish supermarket in Paris days after the Charlie Hebdo shootings in January, needed weapons, he headed to Brussels. There, he swapped cars bought on fraudulent credit cards in France for automatic weapons and ammunition.

In February, a series of raids and shootouts in Brussels and the small town of Verviers rolled up most of a network of Belgians who had returned from Syria and were preparing to launch a series of attacks, possibly on police. But some got away. Abdelhamid Abaaoud, who is now being seen as the organiser of the attacks in Paris, was one.

The current focus on Belgium may disguise a broader and more worrying prospect. Coolsaet, the terrorism expert, said that although the estimates for the numbers of Belgians who have travelled to Syria were accurate, they may not be exceptionally high. Instead, they could indicate that the true totals elsewhere, especially in France, may be considerably greater than currently thought.

“We have good security services who are lucky to work in a small country with small cities. It’s much easier to get an insight into what’s going on here,” he said. “The figures are reliable. Elsewhere, that is not always the case.”