In the wake of Tuesday's terrorist attack in Brussels, which ISIS has claimed responsibility for, there's one big question on everyone's mind: Why Belgium? How did this small European country become a hub and a target for radical extremists?

To find out, I called up Peter Neumann, a professor at King's College London and the director of the International Centre for the Study of Radicalisation. As the name of his organization might suggest, Neumann is one of the world's leading experts on how people in the West come to commit violence in the name of groups like ISIS.

"The security institutions in Belgium are very, very small"

According to Neumann, Belgium isn't just one of many European countries with a homegrown extremist problem; it's the country with the biggest such problem on the entire continent. It's likely, then, that at least some of perpetrators of Tuesday's attacks at least have links to this Belgian radical network (as initial info about the suspects uncovered on Wednesday suggests).

There are at least two reasons why Belgium has this problem, by Neumann's account. First, the country has an especially longstanding and well-organized network of radical Islamist recruiters, making it easier for people to join up there than in other European countries. Second, its police and intelligence agencies are epically undersized, making them incapable of dealing with the past five years' massive surge in jihadist recruiting.

The Belgian state, according to Neumann, mostly turned a blind eye to these problems. So it was "just a question of time until something happened."

What follows is a transcript of our conversation, edited for length and clarity.

Zack Beauchamp: How big of a problem is jihadism in Belgium?

Peter Neumann: It's a big problem.

If you take, as a proxy, the people who have gone to fight in Iraq [and Syria], which are now estimated for Belgium to be 500, that's the biggest number per capita in all of Europe. That's why people like me who study this having been saying, for two or three years, that Belgium is top of the list of places where something might happen.

But it's not only the number of people that have gone. It's also relative to the size of the security institutions that are charged with monitoring them: The security institutions in Belgium are very, very small.

If you'd spoken to intelligence people or police people in Belgium, they would have been the first to admit that their agencies were not built for the number of people that they're supposed to monitor.

There's a huge gap between the size of the threat in that small European country and the institutions. Because of that gap, it was really just a question of time until something happened.

Paris was a warning. [The November] attacks were in Paris, yes, but they were essentially prepared in Belgium. So Belgium is at the top of the list for everyone who studies this phenomenon in Europe.

ZB: How did the problem of radicalism get so out of control in Belgium?

PN: There's a couple of things coming together.

In Belgium, like in France and other countries in Europe, you have these areas in cities that have over the past years, if not decades, become migrant ghettos. You had a lot of issues with social/economic deprivation — the best example of that is Molenbeek, the part of [Brussels] where all these jihadists seem to be coming from.

These are parts of Europe that have been completely abandoned by the state, by the authorities, by even Muslim communities. And for a long time, people were happy with that. They would be leaving us alone, and we would be leaving them alone.

But over the years, this situation festered. Jihadist structures took advantage of that, and basically go about their business almost unhindered. What happened after 2011-'12 is that groups like Sharia4Belgium — a prominent group — went into these places and very systematically recruited large numbers of people.

"You have motivation all across Europe, but people don't always have the opportunity"

The places where those groups operate are the places where most of the foreign fighters are going from, to Syria and Iraq. And they're also the places where we now think those networks are hiding.

It's the combination of having deep social and economic problems, a government that was no longer interested in engaging in those areas, and extremists taking advantage of that vacuum and implanting themselves in those areas. Quite a lot of things have gone wrong in Belgium. Not only in Belgium but also in France.

ZB: Okay, but if Belgium and France have similar problems with neglected and isolated communities, why has Belgium's per capita rate of jihadism gotten so much worse in recent years?

PN: The war in Syria was the cause of this mobilization: That affected Belgium and other European countries. But for some reason — and people in Belgium haven't completely figured it out — it affected Belgium a lot more strongly than other European countries.

If you go to Belgium, that's the first question they ask themselves: "Why us?" I'm not sure I have the perfect answer to that; other European countries are also strongly affected by that, but Belgium seemed to be particularly strongly and particularly suddenly affected.

I think it is correlated with the activities of certain groups — like the one I just mentioned, Sharia4Belgium — which were particularly active in picking up people and channeling them into [radical] structures. Some of them go to Syria, but some of them stay back and become part of the jihadist movement.

ZB: Are there equivalents to Sharia4Belgium — which existed before the Syria war — in other countries?

PN: In some countries, yes, but it was particularly active and particularly strong in Belgium.

Look, there's grievances everywhere. In a lot of countries, people are unhappy, they maybe don't like society particularly much. But not in every place is there something that you can channel your grievance into. And the activities of these groups, and the presence of those groups in particular in Belgium, made it possible for them to channel that grievance in the direction of jihadism.

Whereas in other places, people would have looked around and not found anything. Perhaps they would have been very frustrated but would not have had the opportunity to do something.

So it's about motivation and opportunity, like every crime. You have motivation all across Europe, but people don't always have the opportunity.

ZB: So Belgium had a preexisting network of radical recruiters before the Syria war, who could then really ramp up their activities once the war began. And to make matters worse, the Belgian security forces are unusually weak.

PN: It's a country with a population of 11 million people that, historically, hasn't had a big terrorism problem. It's also a country that's deeply divided within, where you don't really have a functioning federal structure.

All of that contributed to the situation we're having now: We have security agencies that could have maybe coped with, say, 100 people going to Syria, but not 500.

"Politicians in Belgium reacted too late"

Compare this to America. In America, we have a country with a population of 330 million and security agencies that are all over the place. And you have 100 foreign fighters to look after.

In Belgium, a country with a population of 11 million, you have 500. And every foreign fighter comes with people who are connected to him [in radical activity] who have stayed at home. So this is a massive, massive, sudden rise of jihadist activity that security agencies were not able to cope with from 2013 onward.

People in the intelligence community in Belgium were pretty open, saying, "Guys, we are completely overwhelmed. We have no idea what's going on, and it'll only be a question of time until something blows up."

ZB: So why didn't Belgium build up its security establishment in response?

PN: No one saw this coming. [In] 2011-2012, you suddenly have people in large numbers — numbers that the agencies and authorities were not expecting — going to Syria and Iraq. And the threat perception wasn't so great, so the growth of these institutions didn't keep up with the growth of the threat.

It's also very questionable whether you can grow security agencies as fast as the threat. You cannot just say, if the FBI is suddenly surprised by 10,000 foreign fighters in America, that tomorrow we're going to hire 10,000 people for the FBI. It doesn't work like that. You have to grow these institutions organically; people have to be trained.

So you cannot double the size within half a year, but that wasn't even the intention. Politicians in Belgium reacted too late to what was happening on the ground. It was a political failure, in terms of not allowing these agencies to grow.

But the more profound failure was to basically allow this situation to grow in the first place: to not engage with parts of the Belgian population that clearly were being abandoned. You essentially allowed a vacuum to rise in your own country. And that's the root cause of the problem: Where you have a vacuum, that vacuum will be filled.

If you have a vacuum that consists of alienated, marginalized people from migrant backgrounds who are socially and economically deprived, then it is only a question of time. Of when extremists go into that, take advantage, and push their narrative — which is basically that society is against you, and you need to engage in war.