[T]he Fourth Amendment's balance of reasonableness is qualitatively different at the international border than in the interior. Routine searches of the persons and effects of entrants are not subject to any requirement of reasonable suspicion, probable cause, or warrant, and first-class mail may be opened without a warrant on less than probable cause,
Ramsey, supra. Automotive travelers may be stopped at fixed checkpoints near the border without individualized suspicion even if the stop is based largely on ethnicity,
United States v. Martinez-Fuerte, 428 U. S. 543, 562-563 (1976), and boats on inland waters with ready access to the sea may be hailed and boarded with no suspicion whatever.
United States v. Villamonte-Marquez, supra.
These cases reflect longstanding concern for the protection of the integrity of the border. This concern is, if anything, heightened by the veritable national crisis in law enforcement caused by smuggling of illicit narcotics, see
United States v. Mendenhall, 446 U. S. 544, 561 (1980) (POWELL, J., concurring), and in particular by the increasing utilization of alimentary canal smuggling. This desperate practice appears to be a relatively recent addition to the smugglers' repertoire of deceptive practices, and it also appears to be exceedingly difficult to detect. Congress had recognized these difficulties. Title 19 U. S. C. § 1582 provides that "all persons coming into the United States from foreign countries shall be liable to detention and search authorized . . . [by customs regulations]." Customs agents may "stop, search, and examine" any "vehicle, beast or person" upon which an officer suspects there is contraband or "merchandise which is subject to duty." § 482; see also §§ 1467, 1481; 19 CFR §§ 162.6, 162.7 (1984).
Balanced against the sovereign's interests at the border are the Fourth Amendment rights of respondent. Having presented herself at the border for admission, and having subjected herself to the criminal enforcement powers of the Federal Government, 19 U. S. C. § 482, respondent was entitled to be free from unreasonable search and seizure. But not only is the expectation of privacy less at the border than in the interior, see
e. g., Carroll v. United States, 267 U. S. 132, 154 (1925); cf.
Florida v. Royer, 460 U. S. 491, 515 (1983) (BLACKMUN, J., dissenting), the Fourth Amendment balance between the interests of the Government and the privacy right of the individual is also struck much more favorably to the Government at the border.
Supra, at 538.