In Washington v. Texas', 388 U.S. 14 (1967), the U.S. Supreme Court declared the Compulsory Process Clause to be so fundamental to a fair trial and due process that it applied its protections against the states.
Some state statutes barred a person who had been charged as a participant in a crime from testifying in defense of another alleged participant unless the witness had been acquitted. As a result, when the defendant in Washington was tried for murder, he was precluded from calling as a witness a person who had been charged and previously convicted of committing the same murder. Holding that the defendant's right to put on a defense had been violated, the Court noted that the rule embodied in the statutes could not "even be defended on the ground that it rationally sets apart a group of persons who are particularly likely to commit perjury" since the rule allowed an alleged participant to testify if he or she had been acquitted or was called by the prosecution. 388 U.S. at 22-23.