The day after the Brussels attacks, in which 32 people died at Zaventem airport and the Maelbeek Metro station, I travelled to the Belgian capital and found an alarming lack of urgency among the forces of law and order. While some of the press reported that there were armed police on the streets, I witnessed only the lightest of security presences wherever I went. When I visited the suspected “bomb factory” in Schaerbeek, where the hardware for the November attacks in Paris was made, it seemed like any other day in the troubled district.

British officials, by contrast, were much more proactive at St Pancras Station in London before I boarded the Eurostar: there was an increased police presence, as well as enhanced security checks and sniffer dogs. In the UK, intelligence officers have long grumbled about the incompetence of their Belgian counterparts, complaining about a lack of sophistication to their tradecraft and approach. What I saw on my visit to Brussels – the somewhat minimalist approach to security just days after the deadliest terror attack in Belgium’s history – gave me reason to believe this.

The most worrying aspect of the Brussels bombings is the extent to which they expose how European intelligence agencies frequently miss the people they should be catching. The three suicide attackers were known to the security services and were suspected of moving within radical networks, yet somehow they managed to evade arrest for months before launching their attacks.

One of the operatives in the plot, Ibrahim el-Bakraoui, who blew himself up at Zaventem, was suspected of having tried to join Islamic State in Syria. When he arrived in Turkey in June last year security officials there arrested him, but were told by their Belgian counterparts that he was no of interest to them. He was eventually deported to the Netherlands, where he had originally caught the flight to Turkey.

The same is true for the mastermind of the co-ordinated series of terror attacks in Paris, which killed 130 people. Abdelhamid Abaaoud was a known extremist and had appeared many times in Islamic State propaganda. Despite this, he seems to have moved freely back and forth between Belgium and Syria, and is suspected of orchestrating several terrorist plots in Belgium and France in the past two years.

The number of Europeans who have joined militant groups in Syria and Iraq over the past couple of years has taken intelligence agencies by surprise. No one could have foreseen just how quickly or how many would travel out. A large proportion of those who went were “clean skins”: that is, they had no prior history of radical activism and were unknown to the authorities.

The best example of someone like that in Britain is Roshonara Choudhry, who in May 2010 stabbed the Labour MP Stephen Timms during a constituency surgery in the East End of London, as “punishment” for having supported the war in Iraq. The Choudhry case showed just how difficult it is to stop someone who self-radicalises and has no history of troublesome associations.

To miss these plots is one thing, but to let known radicals slip through the net, as happened in Belgium, is quite another. The attacks there have led to soul-searching about why that country was a target for home-grown terrorists nurtured by Islamic State. (Two of the attackers, the el-Bakraoui brothers, were born in Brussels; the third jihadi, Najim Laachraoui, was raised there.)

Like France, Belgium has deep problems with its Muslim immigrant communities. Parts of the Belgian capital, such as Schaerbeek, have become hotbeds of radicalisation and, like the notorious Molenbeek, are isolated and detached.

Belgium has more foreign fighters per capita than any other country in Europe – roughly 46 per million people. According to a study by the Soufan Group, the New York-based security intelligence consultancy, 74 per cent of the estimated 5,000 European jihadi fighters in Syria and Iraq are from four countries: Belgium, France, Britain and Germany.

Another consequence of the Brussels ­attacks has been renewed debate about refugees and migrants. Some politicians and newspaper columnists insist that refugees are largely to blame for the worsening security crisis in western Europe and that immediate action must be taken to halt the flow. There is a broader discussion to be had about the merits or otherwise of mass migration, though it has nothing to do with security. Almost all of the recent attacks in Europe have been carried out by our own citizens, people who were born and raised in the very countries they now wish to destroy. It is this problem that has to be addressed if we are to mitigate the terrorist threat in Europe.

Much is made of the supposed sophistication of Islamic State’s propaganda, but its strength lies in its simplicity. The argument is binary and primitive, revolving around ideas of social deprivation, racism, identity and acceptance. The group’s messaging tells Muslims to join IS in the self-proclaimed caliphate because issues of race and identity melt away there. It offers an alternative identity, one that transcends culture and ­geography by anchoring itself in confessional and kindred terms by which all Muslims belong to a fraternity of the faithful, through the umma.

There is little religious or ideological messaging from IS (by contrast, al-Qaeda has always spoken a much more religious language and offered a greater ideological challenge). This approach finds a ready audience in those parallel and separate worlds – the Molenbeek or Schaerbeek communes of Brussels, or the French banlieue.

Shiraz Maher is a contributing writer for the New Statesman and a senior research fellow at King’s College London’s International Centre for the Study of Radicalisation