September 11, 2001, stands as a critical pivot point in history, one that put the threat of terrorism in the national spotlight and demanded immediate expertise in national security. Yet, as new as the issue may have seemed to many observers, this demand for legal, political, and technical attention to national security reflected a longer-term set of profound changes. The fall of Communism and the end of the Cold War ushered in a new era of geopolitics, ending a past marked largely by alliances between—and rivalries across—nations and beginning an era of global risks from actors not easily defined by, or constrained within, state borders. The collapse of the Soviet Union left nuclear weapons and materials in unstable countries and unprotected facilities, providing attractive targets for rogue states and terrorist groups. This precarious situation has been exacerbated by the growing ambitions of Iran and North Korea. A revolution in digital and information technologies has produced innovations that not only afford new intelligence capabilities for government authorities but also create new forums for terrorist communication, recruitment, and training. Mass migrations of people due to economic hardship, ethnic and religious conflicts, and climate change increase the likelihood of disputes over basic resources, further adding to the risk of violence. Alongside these emerging threats, many traditional security concerns remain salient, including human rights violations in China, tensions between India and Pakistan, and the menace of international piracy.

Devising responses to threats from each of these sources produces its own set of complications and complexities for any nation and for the international community. The U.S.-led invasions of Iraq and Afghanistan called into question settled precepts of warfare, including the use of preemptive force, the relevance of the Geneva Conventions, the distinction between enemy combatants and civilians, the role of private military contractors, and the function of the United Nations Security Council. The global war on terror challenges domestic law, practices, and politics inside the United States, especially regarding port and border security; coordination among federal agencies; individual privacy and government secrecy; and collaboration between federal, state and local governments. Assessing and responding to national security threats requires new bridges across the public and private divide, for effective security strategies must bring together government actors, private companies, and nongovernmental organizations in direct or indirect partnerships. New legal questions accompany each of these challenges; such questions cast doubt on once- settled legal doctrines and thus present an opportunity for the forging of new areas of law, which in turn raises an array of legal and policy concerns. Lawyers and legal scholars are discovering that in this new age, national security issues cannot be broken down and analyzed as isolated topics. National security concerns implicate domestic and international dimensions, legal and policy issues, and technological and philosophical problems, and, indeed, call for attention to all of these elements as a whole.