This article was written a long time ago, when Bin Laden was still alive. It is, however, just as relevant today with ISIS as it was back then:
Terrorist Strategy 101: A Quiz The two most relevant points:
1) "The first and biggest obstacle to your victory [your = the terrorist] is that the vast majority of the people who sympathize with your issue are not violent extremists. They may agree with you in principle. They may even sound like violent extremists late at night over their beverage of choice. But when the hammer comes down, they won't be there."
2) "In radicalizing your apathetic sympathizers, you have no better ally than the violent extremists on the other side. Only they can convince your people that compromise is impossible. Only they can raise your countrymen's level of fear and despair to the point that large numbers are willing to take up arms and follow your lead. A few blown up apartment buildings and dead schoolchildren will get you more recruits than the best revolutionary tracts ever written."