In Almeida-Sanchez v. United States (1972), the U.S. Supreme Court ruled that warrantless searches of vehicles by the Border Patrol, conducted without probable cause or consent, were unconstitutional under the Fourth Amendment. The case involved a Mexican citizen who was stopped 25 miles away from the border and his vehicle searched without a warrant, leading to discovery of marijuana. The government argued these searches were necessary for immigration control but did not have specific evidence against Almeida-Sanchez himself at time of search. In a 6-3 decision, Justice Potter Stewart delivered majority opinion stating that such roving patrols violated privacy rights protected by Fourth Amendment unless officers had reasonable suspicion to believe that an immigration law has been broken.

In the dissenting opinion for Almeida-Sanchez v. United States, Justice Blackmun argued that the majority's decision was too restrictive and would hamper law enforcement efforts to control illegal immigration. He contended that roving patrols were necessary due to the vastness of U.S borders and their inability to be effectively patrolled by fixed checkpoints alone. Furthermore, he believed that these searches should not require probable cause or a warrant as they are aimed at protecting national security rather than criminal prosecution. The justice also expressed concern about the potential implications of this ruling on other types of border searches such as those conducted at airports or seaports which could now potentially be deemed unconstitutional under this new precedent.