In the 1959 case of Henry v. United States, the Supreme Court ruled that law enforcement officers must have probable cause to stop and search a vehicle without a warrant. The case arose when federal agents arrested Robert Henry and his accomplice on suspicion of theft after they were seen driving in an area known for recent burglaries with merchandise visible in their car. Upon searching the vehicle, agents found stolen goods leading to their arrest and conviction. However, upon appeal, it was argued that there wasn't sufficient probable cause for stopping or searching the car initially as mere suspicious behavior doesn't constitute probable cause under Fourth Amendment protections against unreasonable searches and seizures. The Supreme Court agreed with this argument overturning Henry's conviction by stating that "a search is not made legal by what it turns up; illegality is determined by whether there was justification before the search." This ruling reinforced constitutional safeguards requiring law enforcement officials to establish reasonable grounds based on factual evidence before conducting warrantless searches.

In the dissenting opinion for Henry v. United States, Justice Brennan argued that the majority had misinterpreted Carroll v. United States and Brinegar v. United States by suggesting these cases established a "probable cause" standard for warrantless searches of vehicles in motion. He contended that both cases actually required an exigency beyond mere probable cause to justify such searches - namely, the impracticability of obtaining a warrant due to either the mobility of cars or other pressing circumstances like imminent destruction or removal of evidence. In this case, he believed no such exigency existed because federal agents had ample opportunity to secure a search warrant before stopping and searching Henry's car based on their prior surveillance and knowledge about his illicit activities.