The Amaq News Agency, a propaganda arm of the Islamic State (or ISIS), has claimed responsibility for today's attacks in Brussels. The claim is hardly surprising: The Islamic State has had Belgium in its crosshairs since at least 2014.

Belgian and European authorities knew this day was coming and they couldn't stop it. Beyond the loss of life, that is the most troubling aspect of today's bombings.

Europe's counterterrorism defenses have cracked because there are simply too many threats to track. An unnamed Belgian counterterrorism official made a similar point during a recent interview with BuzzFeed News. Citing this official, BuzzFeed reported that "virtually every police detective and military intelligence officer in [Belgium] was focused on international jihadi investigations."

"We just don't have the people to watch anything else and, frankly, we don't have the infrastructure to properly investigate or monitor hundreds of individuals suspected of terror links, as well as pursue the hundreds of open files and investigations we have," this same official told BuzzFeed.

As a result, we have reached a point where known terrorists are slipping through the West's defenses. And jihadist networks targeted by counterterrorism officials are still able to carry out attacks even though, in many cases, their members are being hunted.

Consider the story of Abdelhamid Abaaoud. French counterterrorism officials quickly identified Abaaoud as the key ringleader of the November 2015 massacres in Paris. Several members of the Islamic State assaulted locations throughout the city, killing 130 people and wounding dozens more.

Abaaoud was a known threat in both France and Belgium. French authorities suspect he was involved in two previous plots: including an attack on a Paris-bound train and another plot against a church in the suburbs of Paris. In January 2015, Belgian officials killed two members of Abaaoud's cell during a raid on their safe house in Verviers.

Abaaoud didn't keep a low profile, despite the fact that European officials knew who he was. Just weeks after the raid in Verviers, in February 2015, the Islamic State's Dabiq magazine published an interview with Abaaoud. Dabiq described Abaaoud as "a mujahid being pursued by Western Intelligence agencies for his jihad in Belgium."

Abaaoud admitted in his interview that, along with two accomplices, he had traveled to Europe "in order to terrorize the crusaders waging war against the Muslims." He taunted European officials for being unable to stop him during his trips into the heart of Europe.

Abaaoud added that Belgium was a target because the country "is a member of the crusader coalition attacking the Muslims of Iraq and Shām [Syria]."

In its claim of responsibility earlier today, Amaq used similar language, saying Belgium is "a country participating in the international coalition against the Islamic State."

Belgian authorities certainly knew there was an ongoing threat to their citizens' security. They have repeatedly launched counterterrorism raids for the better part of the last two years. Last week, Belgian officials arrested Salah Abdelslam, an Islamic State operative who is accused of helping Abaaoud and the other Paris attackers. Abdelslam was perhaps the most wanted man in Europe, yet he managed to live for several months undetected in a Brussels neighborhood.

There are still many details to be worked out. But the early suspicion is that a network tied to Abdelslam and Abaaoud carried out today's attacks. We shouldn't be surprised if that turns out to be correct.

The jihadists said they were going to attack Belgium. Belgian authorities tried to stop them. But the jihadists succeeded anyway.

Until a real strategy is put in place to defeat the Islamic State and al Qaeda, Europe will continue to play defense. And, unfortunately, the jihadists will continue to kill the West's citizens.