"Our national security system is principally designed to deter and defeat state-based threats, which it does with great success. But it is precisely because of our overwhelming capability to overcome conventional threats that our enemies will attempt to defeat us through other means. That fact, along with the increasing danger of transnational, regional, and substate threats has made it clear that the most dangerous threats the United States will face in the 21st century are as likely to come not from other states but from within states, and principally from within weak or poorly-governed states. The Westphalian world order is being challenged by a new form of insurgency -- transnational, distributed, networked, and ideologically driven -- that is most graphically represented by al-Qaeda, a global Islamist movement that seeks to replace secular governments with theocratic Islamic regimes hostile to liberal values and secular democracy. The War on Terrorism is a national response to high-profile violence conducted by small but potent movements of violent extremists seeking to destabilize and disempower national governments. Their targets are nations whose policies and presence in Islamic countries prevents the realization of their vision of a global 'virtual Caliphate' of Islamic governments in predominantly Muslim countries around the world. Other non-state organizations using terrorism or organized political violence for local or global ends that are inimical to US national interests adds to the challenge."
U.S. Government Interagency Counterinsurgency Initiative: http://www.usgcoin.org/index.cfm