This book has involved scholars thinking historically about terrorism. In relation to the four main areas of understanding in the field – definition, causation, consequences and appropriate response - what can we therefore say that we know, and what should we prioritise next in our research? This chapter will identify some of what the contributors themselves have valuably argued, and it will consequently have a historical dimension. But it will also relate such ideas to wider understandings, findings and agendas, recognising that the study of terrorism is and should be collaborative between disciplines.

A definitional modification has had the effect of greatly magnifying the perceived importance and frequency of terrorism. The United States failed in its military interventions in Libya and in the Syrian civil war, both of which replaced coherent if unpleasant regimes with chaos and murderous disorder. There was, however, a successful campaign against Islamic State, or ISIS, or ISIL, an especially vicious, ultimately self-destructive, insurgent group that had a genius for making enemies and owed its initial successes in 2014 primarily to the often-monumental incompetence of the US-trained Iraqi army. However, as with al-Qaeda after 9/11, ISIS scarcely presented a challenge to global security, inspired near-total hostility in the area, and was soon pushed back. In defense and in decline, ISIS relied primarily not on counteroffensives, but on planting booby traps, using snipers, and cowering among civilians, and the costs for defeating it might have been lower if the methods to do so had been more measured. The strategy against ISIS worked because of a couple of features not likely to be found in many other conflicts: local forces were prepared to do the fighting and dying, and ISIS inspired existential angst in the US public.