In which case was the "exclusionary rule" first enforced against state officials?

The exclusionary rule was first enforced against state officials in the case of Mapp v. Ohio. This landmark decision, issued by the U.S. Supreme Court in 1961, established that evidence obtained in violation of the Fourth Amendment, which protects against unreasonable searches and seizures, cannot be used in state courts. Prior to Mapp v. Ohio, the exclusionary rule had only been applicable to federal prosecutions.

The significance of this case lies in its reinforcement of the principle that constitutional protections should be uniformly applied at both state and federal levels. Through Mapp v. Ohio, the Court incorporated the Fourth Amendment to the states through the Due Process Clause of the Fourteenth Amendment, ensuring that individuals' rights would be safeguarded against unlawful searches and seizures, regardless of whether the prosecution occurred at the state or federal level. This case set a critical precedent and expanded the scope of constitutional protections in the American legal system.