I think one of the most important things that we can do when faced with these types of events, but which usually fails to happens, is for us to humbly drop our assumptions, and to dig in to try understand the deeply complex histories and motivations that led to the attacks. If we don't understand why something is happening (and no, it's not because "ISIS is evil incarnate", as some try to proclaim), how we can ever hope to stop it, or more importantly, prevent similar/analogous conditions that result in this type of outcome from arising again? Due to the complex cultural, historical, economic, and psychological factors at play, there is no "easy fix" that actually solves the problem in the long run.

Edit: I don't claim to have a rich understanding of the issue, but I appreciated the nuanced analysis in this article:

http://www.theatlantic.com/magazine/archive/2015/03/what-isi...