Le Sassier v. Kennedy was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner who was being held in a federal prison. The case arose when a prisoner, Le Sassier, was held in a federal prison in Louisiana. Le Sassier sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the power to interfere with the federal government's authority to imprison individuals. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a person's detention, and not to challenge the conditions of the detention. The Court's decision in Le Sassier v. Kennedy established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts cannot interfere with the federal government's authority to imprison individuals.

Justice Field delivered the dissenting opinion in Le Sassier v. Kennedy, arguing that the majority decision was wrongfully decided and should be overturned. He argued that under California law, a married woman had no right to sue for damages caused by her husband's negligence without joining him as a party to the suit. The plaintiff in this case was not joined by her husband because he had abandoned her before she brought suit against Kennedy for his negligent act of driving an ox cart into hers and causing damage to it. Justice Field maintained that since there were no laws allowing a wife to bring such an action independently from her husband, then any judgment rendered on behalf of the plaintiff would be voidable at law due to lack of legal capacity or authority on part of the plaintiff herself. Furthermore, he argued that even if such actions could have been taken independent from one's spouse, they still must comply with state statutes which require both parties' consent prior to bringing suit; thus making any judgment given without said consent invalid regardless of whether or not it is legally permissible otherwise. In conclusion, Justice Field believed that since neither condition applied here - i.e., there being no statute permitting independent suits nor having both parties' consent - then any judgment rendered would be null and void according to California law and should therefore be reversed upon appeal