A successful strategy to combat terrorism should incorporate at least four elements: offense (efforts to capture and kill terrorists and disrupt their networks); defense (the physical protection of the United States and the global protection of WMD materials); preventive diplomacy (efforts to address the root causes of terrorism, and thus mitigate both the duration of conflict as well as the potential number of persons willing to attack the United States); and, a response and recovery capacity to respond to homeland incidents regardless of cause. Such a strategy should employ the full array of national security tools and, through employment of these tools, offer geographically and functionally concentric opportunities to prevent and deter attack.

This chapter considers legal aspects relating to the element of defense, that aspect of national security known after 9/11 as homeland security. The chapter starts by reviewing the nature of the homeland threat. However, part of the difficulty in reaching agreement on the elements, costs, and benefits of a homeland security plan derives from disagreements on the nature of the threat. In some cases, disagreements on implementation, in fact, reflect underlying disagreements on the risk presented. Therefore, the threat is defined up front, from which the homeland security regime should follow.

Included in the discussion of the threat are facts and figures that should give the reader a sense of the scope of the defensive problem. However, the facts are evolving as the United States improves its security.