This history behind the landmark decision in Mapp v. Ohio (1961) is as follows: Police officers in Cleveland requested admission to enter a home to look for a fugitive who was allegedly hiding there. The police had also received information that a large amount of policy paraphernalia was hidden in the house. So, without a warrant, the police officers forced their way into the house, found obscene materials, and convicted Dolree Mapp in the state courts of possessing these obscene materials even after an admittedly illegal police search of her home for a fugitive. Mapp appealed her conviction on the basis of freedom of expression.
The case was brought to the Supreme Court which asked: Is evidence obtained in violation of the search and seizure provisions of the Fourth Amendment admissible in a state court? That was the question presented in the case. In other words, were the materials which were confiscated protected by the first amendment?
In a 6-3 decision, the Courts answered: No. Evidence obtained in violation of the provisions in the Fourth Amendment is not admissible in a state court. Basically, the Court brushed aside the First Amendment issue, and declared that "all evidence obtained by searches and seizures in violation of the Constitution is, by the Fourth Amendment, inadmissible in a state court."
The Supreme Court reasoned that Mapp had been convicted on the basis of illegally obtained evidence, hence the conviction is not admissible. The Court believed that since the Fourth Amendment's right of privacy has been declared enforceable against the states through the Due Process Clause of the Fourteenth Amendment, it is enforceable against them by the same sanction of exclusion as is used against the Federal Government. Therefore, all evidence obtained by searches and seizures in violation of the Constitution is, by that same authority, inadmissible in a state court.
This was also, a landmark decision by the Supreme Court. It was also a very historic and controversial decision. Consequently, the Wolf v. Colorado (1949) decision was overruled, and reversed. Another consequence was that now the Fourth Amendment's right of privacy has been declared enforceable against the states through the Due Process Clause of the Fourteenth Amendment. It also placed the requirement of excluding illegally obtained evidence from the courts at all different levels of the government. It also caused the Court to figure out how to determine how and when to apply this exclusionary rule.
Sources:http://oyez.nwu.edu/cases/cases.cgi and notes from Government class.