The vicious attack on the Brussels airport and metro underscores the futility of focusing on the Syrian "Caliphate" as the epicenter of terrorism: as I've been saying in this space since 2001, the snake has no head. Both al-Qaeda and now ISIS are protean entities with a vast geographical spread, and what the Brussels attack – and, before it, the Paris attack – show is that they have successfully colonized Europe.

If the "Islamic State" proclaimed by ISIS was defeated and eliminated tomorrow, the terrorist and criminal networks that pulled off the Brussels attacks would still exist.

The population of Brussels is nearly 25 percent immigrants from Muslim countries, primarily Morocco and Algeria. And as it turns out the two brothers who were the core of the ISIS cell were habitués of the now notorious Molenbeek neighborhood, which consists primarily of the descendants of immigrants who settled there decades ago. Poor, and beset by petty crime, it is a pool in which terrorist recruiters fish with much success. The Syrian civil war has become a cause that attracts young toughs with no prospects, who are looking for some sense of meaning – and a way to express their alienation from the larger society in which they live. Molenbeek was also the base for those who planned and carried out the Paris attacks – it is, in effect, a general headquarters for ISIS to carry out its European operations. Salah Abdeslam, the chief planner of the Paris attacks, fled there and found sanctuary for four months before being caught.

In short, the problem of terrorism in Europe is an internal phenomenon, not something that comes from the outside. The Europeans imported it – and, as Germany's welcoming of hundreds of thousands of refugees from the war-torn Middle East dramatizes, they are continuing to import it. Now they are living with the consequences.

In response, various right-wing populist parties have emerged in Europe that focus on stopping immigration from Muslim countries: in France, Britain, and Germany the rise of the anti-immigration movement has liberal elites in a panic. And yet these movements are for the most part exercises in futility, because that horse is already out of the barn. France, for example, is not going to deport the millions of North African Muslims who have lived in the country for a generation and more: they are French citizens. The same goes for Britain and all the former empires of Europe whose colonial adventures brought in large numbers of the colonized. Now they are learning – too late – that colonialism is a two-way street.

What Brussels also showed is that the universal surveillance championed by the War Party as a necessary corollary of the "war on terrorism" would not have stopped the attacks: the ISIS cell consisted of two brothers, which not only ensured against infiltration but also made it next to impossible for any but the most intrusive surveillance to have had any effect. Indeed, the key to stopping the attacks was intelligence – which the Belgian authorities ignored. It turns out that Brahim el-Bakraoui had been deported from Turkey and the Belgians had been warned he was dangerous. They ignored the warning.