It is still unclear exactly what Brussels has faced just prior to Easter. The random nature of the date and targeting suggests a plot that may have been brought forward, while the scale of the attack suggests it must have been in the pipeline for some time. The Isis network, also linked to November’s Paris attacks, has claimed responsibility. The bigger issue, however, is not who is to blame for this atrocity but rather how much Europe will warp to address an acute terrorist threat, with cells apparently able to launch large-scale atrocities on an increasingly regular basis.

The first questions raised will focus on Belgium’s response to the problem on their home ground. Authorities may have scored a victory by capturing Salah Abdeslam, one of the Isis-aligned plotters linked to the Paris attacks, but they missed a network planning an atrocity with heavy weapons and explosives. This suggests gaps in the understanding and surveillance of the terrorist threat. Given that Brussels sits at the political heart of Europe, this points to a problem that can no longer be described as Belgian alone.

While for some the terrorist atrocities in Paris was a wake-up call, for security forces it had been expected for a while. Terrorist groups, from al-Qaeda to Isis, have long sought to launch a terrorist attack in the style of the 2008 Mumbai attacks, and a string of plots have been disrupted or launched from a francophone network emanating from Brussels. The Paris attack was the realisation of these fears from a depressingly predictable place.

The networks of radicalised individuals with links to Isis have grown as the group continues to hold sway on the battlefield and send back people and plots to their original bases in western Europe. Given the tempo of attacks and the ease with which the networks appear able to acquire weapons and move freely around the continent, Europeans will ask themselves how much longer they will face this threat. Is this the start of a regular diet of such atrocities or the breaking of a wave? Given that terrorist groups have been able to launch three big, ambitious plots in Europe in the past year and half, the sense will be that we are in the thick of this threat with no end in sight.

The choice of targets is predictable. Terrorist groups have long fetishised aviation as a target, both as a way of visibly lashing out against the globalised political establishment but also for the high impact. Mass transport systems by their very nature have to be open to the public, which makes them tempting targets as they offer an easy opportunity to strike at the heart of a society. Questions will be asked about ramping up security levels but this will bring costs and further inconvenience to the daily lives of citizens. Think of the ramifications of a plot in 2006 where a cell planned to use liquid bombs on a series of transatlantic flights. Liquids are still banned on aircraft today.

The Brussels attacks will also play badly against the backdrop of Europe’s migration crisis. It will not be entirely surprising if elements close to the recent attacks found ways of slipping into the country alongside refugees from the Middle East. An already tense situation in Europe will grow more fraught, and this will have inevitable political ramifications too.

This is the biggest problem with which security planners will have to contend. It is often said that the best response to a terrorist threat is to keep calm and carry on. This is sage advice but in the face of a network that appears able to strike with impunity, and a political environment growing more toxic by the day, it will be ever harder for security forces and politicians to ensure that Europe maintains its values in the face of the terrorist threat from within.