Writing for the Court, Justice William Brennan held that the Fourth Amendment prohibits the police from making a warrantless, nighttime entry into a suspect’s home to arrest them for a civil, nonjailable traffic offense. Brennan emphasized that the “physical entry of the home is the chief evil against which the wording of the Fourth Amendment is directed,” meaning that warrantless searches and seizures inside a home are presumptively unreasonable. He established that before the government may invade this sanctuary, the burden is on the state to demonstrate “exigent circumstances that overcome the presumption of unreasonableness that attaches to all warrantless home entries.” He noted that “[w]hen the government’s interest is only to arrest for a minor offense, that presumption of unreasonableness is difficult to rebut, and the government usually should be allowed to make such arrests only with a warrant issued upon probable cause by a neutral and detached magistrate.”

Brennan explained that because warrantless home entries represent such a severe intrusion, the application of the exigent-circumstances exception “should rarely be sanctioned when there is probable cause to believe that only a minor offense... has been committed.” Brennan noted that “an important factor to be considered when determining whether any exigency exists is the gravity of the underlying offense for which the arrest is being made.” To determine the gravity of the offense, Brennan explained that courts must look to statutory penalties, as “the penalty that may attach to any particular offense seems to provide the clearest and most consistent indication of the State’s interest in arresting individuals suspected of committing that offense.” Thus, if a state chooses to classify an offense as a civil, noncriminal violation with no possibility of imprisonment, it signals a minor state interest insufficient to justify a warrantless home arrest.

Turning to Welsh’s case, Brennan rejected each of the State’s claimed exigent circumstances. Brennan found that the hot-pursuit doctrine didn’t apply because there was “no immediate or continuous pursuit of the petitioner from the scene of a crime,” and there was little threat to public safety since Welsh had already arrived home and was no longer driving. He further found that the State’s interest in preventing the destruction of evidence by capturing Welsh’s blood-alcohol level before it dissipated didn’t justify the intrusion, explaining that since Wisconsin classified the offense as a “noncriminal, civil forfeiture offense for which no imprisonment is possible,” the State’s minor interest in prosecution was insufficient to overcome the presumptive unreasonableness of a warrantless entry. Ultimately, Brennan concluded that to allow a warrantless entry under these circumstances “would be to approve unreasonable police behavior that the principles of the Fourth Amendment will not sanction.”