Complex sentence: The Supreme Court reasoned that the Due Process Clause of the Fourteenth Amendment made the right to be able to compel defense witnesses to testify necessary for a defendant's "due process" rights to fair proceedings, which applies to the states.
Split into simpler sentences: The Supreme Court reasoned that the Due Process Clause of the Fourteenth Amendment required this right of being able to compel defense witnesses to testify as essential to having a fair trial. In this way, it was necessary for a defendant's "due process" rights, the right to have fair proceedings, which is a rule that applies to the states.