In addition to the permanent seat of government, established by 1800 in the District of Columbia, the Constitution gave Congress exclusive legislative authority over certain federal installations. Like the federal district, the purpose of this grant was to accommodate and guarantee the independence of both federal and state sovereignties. As Justice Joseph Story noted in his Commentaries on the Constitution of the United States:

The public money expended on such places, and the public property deposited in them, and the nature of the military duties, which may be required there, all demand, that they should be exempted from state authority. In truth, it would be wholly improper, that places, on which the security of the entire Union may depend, should be subjected to the control of any member of it. The power, indeed, is wholly unexceptionable; since it can only be exercised at the will of the state; and it is therefore placed beyond all reasonable scruple.

Federal “enclave” jurisdiction, obtained under this provision, must be distinguished from instances where the federal government has obtained only a “proprietarial” interest in a particular building or parcel of land through purchase—although federal authority over such areas may turn out to be nearly as broad under the Territories and Property Clauses (Article IV, Section 3, Clause 2).

Enclave jurisdiction may come into effect as a result of a federal reservation of legislative authority over an area at the time a state is admitted to the Union, or based upon a particular cession by the state to the federal government of that authority. Federal enclave jurisdiction may apply to individual buildings, or parts of buildings (such as the U.S. Customs House, and the northern portion of the U.S. Mint, located in Denver, Colorado), or to vast territories (such as the 200-square-mile Camp Pendleton in California). Federal enclaves include such varying installations as the National Institutes of Health in Bethesda, Maryland, and Cape Canaveral, Florida, while “other needful buildings” includes locks, dams, federal courts, customs houses, post offices, and “whatever [other] structures are found to be necessary in the performance of the functions of the Federal Government.” James v. Dravo Contracting Co. (1937).

The federal government began regulating federal enclaves from the start with the Federal Crimes Act of 1790. Beginning in 1825, Congress began applying state law to crimes within federal enclaves where federal law was silent. These Assimilative Crimes Acts continue to the present day, including one passed in 1948 that adopted state laws not only then existing but any future state law that may be applicable. In 1958, the Supreme Court validated Congress’s adoption of future state laws, reversing a lower court ruling holding that the 1948 Act was an improper delegation of Congressional authority to the states. United States v. Sharpnack (1958)

The case law dealing with federal enclaves is complex. Such areas are subject to the “special maritime and territorial jurisdiction of the United States,” 18 U.S.C. § 2243; and criminal offenses committed within an enclave are subject to federal prosecution, although the substantive offense may well be grounded in the surrounding state’s law pursuant to the Assimilative Crimes Act, 18 U.S.C. § 13(a). As noted, the 1948 Assimilative Crimes Act provides that where a criminal offense has been committed within a federal enclave and there is no federal law applicable, federal courts will instead apply applicable state law. The applicable state law becomes “absorbed” as federal law.

Outside of the provisions of the Assimilative Crimes Act, the ceding state retains no authority in a federal enclave unless it specifically reserved such rights at the time it consented to the purchase or made the cession. In fact, most states have reserved at least the right to serve state civil and criminal process in federal enclaves, and they may also retain certain regulatory authority. Paul v. United States (1963). At the same time, although a state’s rights vis-à-vis a federal enclave depend upon the terms of the original cession, the Supreme Court has ruled that federal enclave residents are entitled to vote as residents of the surrounding state no matter what the terms of the original cession. Evans v. Cornman (1970).