On Tuesday morning, two explosions rocked Brussels, a small but unusually significant city in Western Europe. It serves as a crucial administrative hub for the European Union and, not coincidentally, sits at the center of the continent's biggest urban centers.

The attacks touch both the political and practical challenges Brussels faces in combating terrorism. Belgium is at the center of European political integration, but it's also a somewhat troubled country, deeply divided over parochial language disputes and featuring state institutions that are very weak by European standards.

One of the bombs detonated at a metro station close to the heart of the city's European Quarter, a symbolic blow to the project of European unity and integration. But the attacks on commuter destinations for an international community of European officials, following Brussels-based attacks on Paris, are also a practical challenge for European integration.

If Belgian law enforcement and intelligence agencies can't do their jobs properly, that's a problem not just for Belgium but for their neighbors in France and Germany and the Netherlands and, ultimately, Finland and Greece and Lithuania too. That adds up to a profound threat to the European project.

1) What is the European Union?

This is a bit of a complicated metaphysical question.

The EU has its origins in the European Coal and Steel Community, which was a pretty straightforward multinational organization that ensured an integrated supply chain for coal and steel between France, Belgium, Luxembourg, the Netherlands, West Germany, and Italy. Over time the organization expanded to include new countries; more importantly, it also expanded to include authority over more and more areas of policy.

At this point, the EU contains 28 member countries and has many of the attributes of a single sovereign state. It has an elected parliament and a permanent bureaucracy. It regulates products and markets, conducts international trade negotiations, and has a nascent diplomatic service. The EU even has its own currency (though not all EU member states use it) — the euro, which has become the world's second most important currency.

Last but by no means least, under the Schengen Agreement most EU member states operate a common visa policy, and one can travel freely from one EU country to the next without showing a passport or stopping at a border patrol. This free movement that allows a person to drive from Portugal through Spain and France into Italy and then onward into Austria is one of the most profound and potent symbols of European unity.

But it also means Europeans face a common security challenge. If Belgian police are persistently unable to surveil and disrupt terrorist cells and plots taking shape in Brussels, then there is very little the French or Dutch or German governments can do to prevent Brussels-based attackers from entering their countries and attacking their citizens.

2) Where is Brussels?

Brussels is roughly in the middle of Belgium, a small country in northwestern Europe wedged between France, Germany, and the Netherlands. Belgium is divided between a Flemish-speaking northern half and a French-speaking southern half. The divides between these regions are extremely important in the country's politics and deeply entrenched in its political institutions.

Geographically speaking, Brussels is located inside the Flemish-speaking area but is administratively separate from both of them, and French is the main language spoken in the city.

Brussels's location puts it very much at an international crossroads of Europe. From Brussels, you could drive north two hours to Amsterdam, three and a half hours southeast to Paris, or four hours east to Frankfurt, or even take a five-hour drive to London through the tunnel beneath the English Channel. This makes the apparent nest of Islamic radicalism in Brussels, and the inability of Belgian authorities to do anything about it, a matter of urgent concern for a variety of nearby countries, most of which are larger and more powerful than Belgium.

3) What does Brussels have to do with the European Union?

Brussels is the closest thing the European Union has to a capital city.

The Maelbeek metro station, where one of this morning's explosions happened, is extremely close to the Berlaymont building, which is the headquarters of the European Commission. The commission is sort of like the EU's Cabinet, and it oversees the EU's permanent bureaucracy. Some of that bureaucracy is also located in Berlaymont, with the rest spread out through dozens of office buildings in the general vicinity. The European Quarter neighborhood also features the headquarters of the European Council (which is the name for when the various EU member state governments meet together) and a building that houses the European Parliament most of the time.

Brussels is not considered an official capital city of the EU, because the parliament's official headquarters is in Strasbourg, France, and the European Central Bank is based in Frankfurt, Germany. But the parliament and its associated staff are in Brussels most of the time, and, crucially, the various lobbying offices for multinational companies that have official EU business to do are generally located in Brussels.

All this also makes the European Quarter neighborhood a center of many hotels and embassies, operating as a kind of modern downtown central business district for Brussels, even though the historic city center is about 20 minutes away, on the other side of Brussels Park.

4) What is Molenbeek?

In the wake of the attacks, attention focused almost immediately on the neighborhood of Molenbeek, which was home to several of the key individuals allegedly involved in the November 2015 Paris attacks, including suspected ringleader Abdelhamid Abaaoud.

The neighborhood is located north and west of the city center (i.e., on the opposite side from the European Quarter), on the "wrong side of the tracks" — in this case, the Brussels-Charleroi Canal that separates it from the city center. In Molenbeek's heyday, the canal made it a convenient center of industrial activity from which goods could be easily shipped to the port city of Antwerp on the coast. It became a crowded 19th-century manufacturing community, but industrial decline began to set in shortly before World War I, and it never really recovered.

In the modern day, it's a residential community essentially divided into two areas. Upper Molenbeek is lower-density and more affluent, while the lower area — which is typically what people refer to when they talk about Molenbeek in this context — is densely populated and features a large population of Muslim immigrants and their descendants.

Social and economic conditions in lower Molenbeek are quite bad, but efforts to improve the situation from either a security viewpoint or an economic one are hampered by Belgium's fractured administrative landscape.

5) Got anything lighter on Belgium for me?

Americans may recall the 1996 hit "Not An Addict" from K's Choice, a band based in Antwerp. Admittedly, a song about heroin addiction isn't all that light.

Belgium is also the setting for the excellent 2008 film In Bruges, a black comedy about gangsters taking refuge in the lovely medieval town of Bruges. It's not all that light, either, but I guess the moral of the story is that Belgium is kind of a dark place.

Belgium's best-known contribution to pop culture is probably the Tintin series of cartoons, of which many of us have beloved childhood memories, though if you look back on it you'll find that they are incredibly racist.

6) What's this "fragmented administrative landscape" you mentioned?

For a country with 11 million residents, Belgium is very complicated.

The basic overview is that Belgians speak two languages: French in the south and Flemish (which is like Dutch) in the north. But there are also some Belgians who speak German, and people who live in Brussels generally speak French even though Brussels is, geographically speaking, located inside Flemish-speaking territory.

This is not handled with a live-and-let-live spirit of bilingualism. Instead, Belgium handles its dual linguistic structure with overlapping levels of decentralization and division.

Specifically:

The country is divided into three regions: Flanders (in the Flemish-speaking north), Wallonia (in the French-speaking south), and Brussels.

The country is also divided into three language communities: Flemish (which is co-extensive with Flanders), Francophone (which is most of Wallonia, plus Brussels), and German-speaking (which is parts of Wallonia).

Flanders and Wallonia are divided into provinces, and Brussels is administered as 19 separate municipalities.

Federal political parties generally limit themselves to one linguistic community. So the Parti Socialiste is social democratic but also Francophone. The Socialistische Partij Anders (SPa) is its Flemish-speaking counterpart. In general, Wallonia is both poorer and more ideologically left-wing than Flanders, and forming viable coalition governments at the federal level requires a complicated balancing act between ideological and linguistic concerns.

As Tim King wrote for Politico Europe, this can make seemingly simple initiatives extremely complicated:

So while Interior Minister Jan Jambon has vowed to clean up Molenbeek, the state structures are aligned against him. He needs the cooperation of the Brussels region and of the French-speaking community to address, for instance, employment agencies and schools. But Jambon is from the Flemish nationalist party N-VA, which has hardly any political capital in Brussels. Brussels is the capital of Flanders, but its population is by and large majority French-speaking. So Jambon’s declaration is viewed with suspicion and in some cases hostility by the Francophone establishment in Brussels.

More broadly, while the international community's No. 1 priority for Belgium is that it not create a hotbed of terrorism at the heart of Europe, the No. 1 issue in Belgian politics is usually tinkering with the complicated fiscal and electoral formulas that preserve the peace between French speakers and Dutch speakers.

7) How did they come up with this kooky country?

It's complicated, but you can think of Belgium as a far-flung consequence of the Protestant Reformation. Once upon a time, the Low Countries were a cluster of 17 provinces ruled by the Habsburg dynasty, which also controlled Spain and Austria. The Habsburgs were the leading Catholic dynasty of the 16th century, whereas many residents of the provinces had adopted Protestantism.

A revolt broke out in the 1560s based on sectarian and other issues.

The resulting war halted in 1609 with the Twelve Years' Truce, which established the de facto independence of what's now the Netherlands. But the southern and western portions of the provinces remained under Spanish (and then later Austrian) control for almost 200 years, until the area was overrun by French forces in 1794 as part of the wars associated with the French Revolution.

Those wars eventually concluded with French defeat, and the victorious powers — most of all England and Russia — wanted to ensure a postwar settlement that would contain French power. They felt that Austria had proven itself incapable of defending a geographically non-contiguous set of provinces, so they reunited the provinces into an enlarged Kingdom of the Netherlands that would presumably be large and prosperous enough to withstand French aggression. That kingdom was created in 1815, but just 15 years later came the Revolution of 1830, when residents of the southern provinces shook off the authority of the central government based in the north.

In the wake of these events, the French government put forward a partition plan arguing that Francophone Catholic territories should be annexed to the Francophone and Catholic nation of France while the Dutch-speaking Catholics could have their own small country under British protection. The other great powers still mistrusted France at this time, and rejected the idea in favor of creating a bilingual Catholic country — Belgium.

8) What does this tangled politics have to do with terrorism?

It's relevant on two levels.

One is that the preoccupation with ethnic and linguistic division makes it difficult for immigrants and their descendants to integrate into Belgian society. All across Europe, nations that have defined themselves in terms that are more ethnic than civic have found assimilation more challenging than in the United States and other settler societies like Canada and Australia. But the situation is particularly problematic in Belgium, because the trend over the past generation has been toward fragmentation of Belgian civic identity into its constituent elements.

The other is that in a practical sense, Belgium has responded to linguistic tensions by pushing more and more authority down to the regional or municipal level. With NATO and the European Union in place to handle big-picture issues of national defense and global trade, Belgium has become a bit of a shell of a central state. Brussels alone used to have 19 separate police departments, one for each of its constitutive municipalities, each with its own mayor. Those have since been consolidated to "only" six police forces, which reduces complexity but introduces the new challenge that each police area is aligned with several different mayors.

Even if Brussels did have an integrated police force, with a population a bit smaller than San Antonio it would simply be a relatively small city in which to operate a sophisticated counterterrorism operation. That's why in the United States we have the FBI and other federal agencies to backstop local authorities on problems of broad significance. But Belgium as a whole has only about the population of Ohio, and its politics are deeply averse to building up new centralized institutions.

The European Union, meanwhile, has a central bank and a space program and a pharmaceutical regulator, but its joint law enforcement arm, Europol, has a tiny staff and no actual executive or law enforcement authority.

9) What's going to happen?

International terrorism arising from Belgium is, in a way, a national security equivalent of the economic crisis that emanated from Greece in past years. They are the result of Europe currently sitting at an unsustainable midpoint in the process of integration.

One possible path forward would be for authorities in France and Germany to conclude that since their national governments have no capacity to address social and economic challenges in Belgium, they need the ability to seal themselves off from problems arising in Belgium. That would mean the return of border controls and passport checks so that Belgium-based plotters can no longer gain easy access to foreign cities.

This would be a significant step back from the promise of European unification. But we saw border checks temporarily reimposed after the Paris attacks last November, and the ongoing migrant crisis in Europe has badly tested the EU's commitment to internal open borders.

But the other possibility would be to follow in the footsteps of the reaction to the debt crisis, where problems caused by partial integration have tended to lead to even deeper integration, with the European Union and European Central Bank now deeply enmeshed in member states' budgetary decisions. In this case, that would entail some centralization of law enforcement powers, through either the creation of a centralized police agency or a formalized process of state-to-state collaboration. Whatever the details, the point would be that if people can hop on a train in Brussels and arrive unimpeded in France or Germany a few hours later, then French and German citizens need equal trust in Belgian law enforcement as they have in their own domestic agencies.

In the specific case of Belgium, pushing power all the way up to the supranational level would likely be more palatable than building a stronger Belgian state, since it wouldn't implicate language issues in the same way. But most European countries have guarded their sovereignty most jealously in the national security terrain, so these ideas would cross significant red lines.

But time and again over the decades, Europe has faced decision points where it either needs to go forward to more integration or backward to less, and the ultimate decision has almost always been in favor of more integration in the end.

President Obama comments on the terrorist attack in Brussels