Imagine a small, prosperous country that has been a destination of immigrants for decades. It’s a country willing to make them citizens, but unable to create nearly enough jobs for them. It struggles with a chronically weak government and a capital city divided into a bewildering number of jurisdictions, policed by multiple agencies.

You would be describing a place where you have a decent chance of evading attention by burrowing deep into an immigrant community; where terrorists might have the time to hatch plots; where the disillusioned sons of migrants might be drawn to violent adventures that seem to give meaning to their lives.

You might be describing any of several European countries, but if you were making a ranking of places with those characteristics, Belgium would be at the top of the list.

The scenes from Brussels on Tuesday were as saddening as they were familiar. Two explosions at Zaventem airport and one at a Metro station killed at least 31 people and wounded hundreds. The Islamic State terror group claimed responsibility, and warned that more was coming.

After Paris and San Bernardino, they were a reminder that you can’t really consider anywhere to be totally safe. But they also posed familiar questions: Why here? Why now?

Belgium has long been recognized as a weak link in Europe’s effort to crack down on jihadi networks. It’s not that the police there are incompetent. They’ve had their successes. Just four days earlier, they captured Salah Abdeslam, the last major suspect wanted in the Paris attacks last November. But there are serious questions whether they should have been better prepared for a response after they picked up Abdeslam. A figure no less than the prime minister, Charles Michel, said Tuesday that what officials feared had come to pass.

So, if authorities were worried about retaliation, shouldn’t someone have noticed the two guys, each wearing a black glove on only their left hand (the speculation now is that it might have hid a detonator) who were pushing carts around the airport Tuesday morning?

It’s likely that Belgian authorities have been simply overwhelmed. Look at the list in this Guardian article of Belgian connections with terror plots. It starts with the assassination of Ahmad Shah Massoud, the most prominent anti-Taliban leader, in Afghanistan just days before the Sept. 11 attacks and continues through the attacks in Paris last January and November. 

As the Guardian makes clear, far and away more people per capita have left Belgium than any other major European country to fight for jihadi groups in Syria and Iraq. Some die there. Some are still there. But according to one estimate, half of those who come back are thought to be potentially violent.

As Germany’s Der Spiegel notes, Belgium has been trying to get a handle on the problem for years. But even Germany, with many more police and intelligence assets, is hard pressed to keep up.

This matters a lot to the rest of Europe. Der Spiegel points out that Abdeslam is thought by investigators to have used the continent’s open borders to drive to Hungary and Germany last year to pick up accomplices. Paris, in particular, has been a target. 

Why does Belgium struggle so much with this?

A lot of people have been looking for answers. Here are a few of them, courtesy of Foreign Policy and Politico:

• According to the Organization for Economic Cooperation and Development (OECD), nowhere in the European Union is the employment gap between natives and foreign-born as high as it is in Belgium. The unemployment rate in the Brussels neighborhood of Molenbeek, where Abdeslam hid out and was finally captured, officially is 30% — and probably much, much higher for young people. Experts say there are few jobs for people who don’t speak both of Belgium’s languages, French and Dutch, or have a university degree. That excludes most Muslim youth. That’s not a guarantee they’ll go fight in Syria, or go on a shooting spree in Europe. But some clearly do.
• The segregation of society creates neighborhoods where jihadis can quite easily be hidden by accomplices, families or friends.
• The national government struggles to reconcile the aspirations of Belgium’s Dutch-speaking Flemish population and the French-speaking Walloons, leaving it chronically unstable – and presumably interrupting both funding and a consistent approach to critical problems like intelligence work. Plus, Brussels is a bureaucratic nightmare. According to the Belgian interior minister, Jan Jambon, the city is divided into 19 districts, each with its own mayor. And the city alone has six different police departments. 

Even so, Belgium itself had largely managed to escape unscathed until Tuesday. There was an attack on the Jewish museum in Brussels in 2014 that killed four people. Perhaps it has been too valuable as a logistics and planning venue. Plus, Paris is easily reachable from Brussels, and an attack on a major European power (one with a strong military and intelligence services) is likely to garner more attention.

Officials are now trying to figure out who, exactly, was behind Tuesday’s attacks and who, exactly, carried them out. It’s likely the bombings were planned well before Abdeslam was picked up. Most experts think they simply were too sophisticated to have been pulled together after he was arrested. However, it’s very possible the timing was moved up to retaliate.

There seems to be an ample supply of potential attackers, capable of acting independently of the Islamic State’s commanders in the Middle East. It’s virtually impossible to say where or when they will try to strike next.  And though police and intelligence agencies will break up many of those attacks — they will not be able to stop all of them.