The Constitution incorporates three structural limitations, or checks, on the exercise of the executive's national security authority. First, the political branches share national security power and they each exercise separate powers as reflected in Articles I and II. Second, the vertical separation of powers, or principle of federalism, divides governmental responsibilities between the federal government, which exercises enumerated constitutional authorities, and the states, to which are reserved the remaining or residual authorities, including, most notably for national security, the police power. Third, the Bill of Rights, the first ten amendments to the Constitution, defines a zone of individual liberty within which the government acts with prescribed and, in some cases, limited authority.

This chapter addresses the separate and shared national security powers of the federal government. There are many books on this topic. Indeed, for some lawyers the study of the separation of powers is the study of government. My objective is to convey the essential ingredients of the law. If I have found new ground, it is in recognizing the role of informal practice in defining the substance, process, and practice of constitutional law. The successful national security lawyer must meaningfully participate in this informal practice as much as he or she participates in the formal practice of constitutional law.

I also recognize (acknowledge may be more accurate) that when the Constitution addresses national security, black-letter law is elusive and constitutional theory pervasive.