Amidst the deluge a sensible explanation why:http://www.politico.eu/article/why-i...error-attacks/

A key passage:
For ISIL, Europe is both a coveted pool of potential recruits and a dangerous counter-model of enduring coexistence. Terror attacks in Europe are intended to generate internal strife and societal tension in the hope of encouraging more young Muslim men (and more than a few women) to join the jihadist cause in Syria and Iraq. And, as importantly, to demonstrate and precipitate the failure of the democratic, European alternative to the jihadist caliphate.
This could fit in a number of threads, but sits here best today.