This was, above all else, an attack on the institutions of Europe, conducted almost certainly by European citizens who hate those institutions, whose loyalties are with a terrorist army that sees itself at war with Europe.
Tuesday morning's bombs struck at the most populated places on the pathway into the political capital of Europe: the Brussels airport, and the Maelbeek metro station, located on the Rue de la Loi exactly where it opens into two sprawling complexes: Berlaymont, where the European Union's executive is based, and Justus Lipsius, home to the European Council.
The symbolic and practical significance of the attack was noted almost immediately by European leaders: "Terrorism struck Belgium, but it was Europe that was targeted," French President François Hollande said in a speech early Tuesday afternoon. The German Foreign Minister, Frank-Walter Steinmeier, said the attacks "aim at the heart of Europe."
The Syrian- and Iraqi-based terrorist group that calls itself Islamic State claimed responsibility for the attack, which was almost certainly carried out by a network of European men, headquartered in a Moroccan neighbourhood in eastern Brussels, who have been responsible for a series of increasingly spectacular terror atrocities in Paris and Belgium.
Tuesday's attacks occurred only four days after Belgian police used a heavily armed siege to arrest a key figure in that circle, Salah Abdeslam, a young Belgian man of Moroccan ancestry who was a key organizer behind November's terrorist massacre in Paris as well as other atrocities.
Mr. Abdeslam declared while in custody that "he was ready to restart something from Brussels" using his Brussels-based terror cell, according to an interview on Sunday with Belgian Foreign Minister Didier Reynders, who warned that further attacks could be imminent.
Nevertheless, the ability of these IS-affiliated European groups to carry out bloody attacks during a full-scale security crackdown has shocked Belgian and European authorities and revealed serious weaknesses in their intelligence and security services. Not since the worst days of the Irish Republican Army's late 20th-century bombing spree has a European terrorist cell managed to carry out an attack on a major political target while that target is under maximum security alert.
In January, the EU's counterterrorism centre (a division of its policing network Europol) warned that they have "every reason to expect that ISIS will undertake a terrorist attack somewhere in Europe again … intended to cause mass casualties amongst the civilian population." Its intelligence concluded that the attacks were likely to come from European citizens loyal to IS, not from Syrians or Iraqis sent to Europe.
Increasingly in recent months, IS has portrayed Europe as its primary target, partly as a consequence of the terrorist group's continuing campaign to stop Syrian refugees from fleeing to the West, and in part because the political, rhetorical and military battle between Europe and the jihadists has intensified.
In February, the EU parliament declared IS's ethnic cleansing actions in Syria and Iraq a legal genocide. Two weeks ago, the EU struck a deal with Turkey to manage the flow of Syrians in exchange for a faster pathway to eventual EU co-operation or even membership. And the EU and its members have provided support to Kurdish forces in Iraq and Syria in their battle against Bashar al-Assad's army and Islamic State's insurgency.
As a result, IS forces have shifted their attention in Europe away from trying to persuade young Europeans to move to Syria and join their fight (though this also continues) but instead, often to urge them to stay in Europe and wage war against the continent's symbolic and political centres, as retaliation for Western presence and military action inside what it sees as its caliphate.
It is likely that the Brussels-based IS cell, which has been behind the Paris attacks last year at Charlie Hebdo in January and in numerous locations in November, the Jewish Museum attack in Brussels and a number of other attacks, had been preparing some sort of spectacular EU-focused attack for some time. But the arrest of Mr. Abdeslam meant that they had to attack immediately before they themselves were raided.
The attack comes at a moment when the European institutions are both more strained than ever – and more necessary. The failure to co-operate around a refugee solution has darkened European relations, and attacks like Tuesday's have led to repeated temporary closures of the continent's normally open borders.
But as French, German, British and Belgian leaders repeated Monday, it has also become an instrument of counterterrorism cooperation that is increasingly necessary – and one that will therefore continue to be a target.
Doug Saunders