In a globalized world with a high volume of cross-border flows, transnational terrorism is a global public bad, while action to control or eliminate it is the quintessential global public good. As such, antiterrorism efforts yield nonrival benefits – enhanced security – received by all at-risk countries. The formation of far-flung terrorist networks has greatly increased the spatial dispersion of benefits derived from measures taken against these networks. The theory of public good supply teaches that as the dispersion of these measures’ benefits increases, their underprovision worsens as providers fail to take account of the benefits that their efforts confer on others when deciding upon antiterrorism actions (see Chapter 4; Sandler, 1997, 2004).

The sheer volume of cross-border exchanges of all types makes it possible to monitor but a small fraction of them, thereby affording opportunities for terrorists to move personnel and equipment internationally. For example, well over a half-billion people cross US borders annually. In 2003, 130 million motor vehicles, 2.5 million rail cars, and 5.7 million cargo containers transited US borders (White House, 2004, p. 165). These transit numbers have grown annually since then. The Department of Homeland Security (DHS) has a daunting task, with 5,000 miles of Canadian border, 1,900 miles of Mexican border, and 95,000 miles of coastline to protect against terrorist and other threats (DHS, 2009, p. 70). According to DHS, 625 million airline passengers were screened in US airports in 2009. The combination of globalization and technological advances means that even the most secure borders may be penetrated by determined terrorists who utilize technologies (for example, communication advances) and apply innovative methods to circumvent security upgrades. Terrorists weigh relative risks to identify the least secure venue or weakest link at which to stage their attack against a targeted nation’s assets. Thus, as we have seen in Chapter 2, most attacks against US interests occur outside the United States, where defenses are weaker. The networking of terrorists facilitates their ability to identify vulnerable targets and exploit governments’ failure, except episodically, to act in unison.