The tragic attacks on the World Trade Center and the Pentagon on September 11, 2001 have made it all too clear that wars do not have to be declared and threats do not have to be overt. It is brutally clear there is a wide spectrum of potential threats to the U.S. homeland that do not involve the threat of overt attacks by states using long-range missiles or conventional military forces. Such threats can range from the acts of individual extremists to state-sponsored asymmetric warfare. They can include covert attacks by state actors, state use of proxies, and independent terrorist groups. They can include attacks by foreign individuals and residents of the United States whose motives can range from religion to efforts at extortion. Motives can range from well-defined political and strategic goals, to religion and political ideology, crime and sabotage, or acts by the psychologically disturbed. The means of attack can vary from token uses of explosives, cyber-terrorism, car and truck bombs, and passenger airliners to the use of weapons of mass destruction (WMD).

We have already experienced some devastating attacks, but no pattern of attacks on U.S. territory has yet emerged that provides a clear basis for predicting how serious any given form of attack will be in the future, what means of attack will be used, or how lethal new forms of attack will be if they are successful. As a result, there is a major, ongoing debate over the range of threats that need to be considered, the seriousness of such threats, and how the U.S. government should react. Years before the attacks of September 2001, a Government Accounting Office (GAO) report on terrorism summarizes the various views within the U.S. government regarding these uncertainties as follows:

There are three schools of thought on the terrorist threat: (1) some believe the threat and likelihood of terrorist attack is very low and does not pose a serious risk; (2) others believe the threat and likelihood of terrorist attack is high and could seriously disrupt the U.S. national and economic security; and (3) still others believe assessments of the threat and vulnerability to terrorist attack need to be accompanied by risk assessments to rationally guide the allocation of resources and attention. The expert further stated that such risk assessments would include analyses of vulnerability and susceptibility to terrorist attack and the severity of potential damage. According to U.S. intelligence agencies, conventional explosives continue to be the weapon of choice for terrorists. Although the probability of their use may increase over time, chemical and biological materials are less likely terrorist weapons because they are more difficult to weaponize and the results are unpredictable. Agency officials also noted that terrorist’s use of nuclear weapons is the least likely scenario, although the consequences could be disastrous.