Terrorist attacks by Islamic State affiliates and sympathizers over the past year have raised alarms in Europe, but they have not yet reached the frequency Europe experienced in the 1970s, according to the Global Terrorism Database. However, whereas previous waves of terrorism in Europe stemmed from internal conflicts, today's deadly surge is linked to instability outside the continent.

The latest attacks are emerging from the political vacuum left by fallen dictators in the Middle East and North Africa. So, just as there seems to be no end in sight for the violence in Syria, Iraq and Libya, or for Egypt's extreme polarization, or for the fragile security situation in Tunisia and Algeria, there is little reason to believe that attacks in Europe will end anytime soon.

Making matters worse, July's bloody putsch in Turkey — where 270 people were killed and another 1,500 wounded in just a few hours — makes the country an even more attractive IS target. IS feeds off troubled states from which it can draw recruits and launch attacks — either by establishing an "official Province," as in Syria, Iraq, Libya and Egypt, or by supporting secret cells and small combat units, as it has done in Tunisia and Turkey already.