Ex parte John A. Davenport was a United States Supreme Court case in which the court held that an individual could not be arrested and detained without due process of law, even if they were suspected of being involved in criminal activity. The case arose when John A. Davenport was arrested by federal marshals on suspicion of counterfeiting coins and notes, but he had not been charged with any crime or given notice to appear before a magistrate for examination. The Supreme Court found that this violated his Fourth Amendment rights against unreasonable searches and seizures as well as his Fifth Amendment right to due process under the law. As such, it ruled that Davenport must be released from custody unless there is sufficient evidence presented at a hearing before a magistrate for him to answer charges related to counterfeiting or other crimes he may have committed. This ruling established important precedent regarding individuals' constitutional rights against unlawful arrest and detention without proper legal proceedings taking place first

In Ex Parte John A. Davenport, the Supreme Court was asked to decide whether a writ of habeas corpus should be issued in order for an individual to be released from prison. The majority opinion held that the writ could not issue because it would interfere with Congress's power over military matters and its ability to punish those who had violated military laws. However, Justice McLean dissented on this point and argued that while Congress does have authority over such matters, it is limited by the Constitution which guarantees certain rights including due process of law and protection against imprisonment without cause or trial. He further noted that if a person has been wrongfully imprisoned then they are entitled to their freedom regardless of any congressional action taken against them. Therefore he concluded that a writ of habeas corpus should issue in this case as there was no evidence presented showing why Davenport had been arrested or detained in the first place; thus his detention was unconstitutional and illegal under both state and federal law.