Introduction

Terrorism² and insurgent violence have become constant threats in today’s world. Nearly every day, nonstate groups in different countries carry out violent actions, many of which can be characterized as terrorism. The threat of such violence drives ongoing global military action, and the need to protect the U.S. homeland against terrorist attack is a primary shaper of the country’s domestic political agenda.³ The experience of PIRA described above illustrates the importance of terrorist groups’ ability to change and adapt. Faced with a challenge to their operational capability, PIRA shifted, adopting a new attack form that reconstituted the threat they could