In the United States ex rel. McCann v. Adams case of 1943, the Supreme Court ruled on whether a defendant could waive their right to trial by jury in federal criminal cases. The petitioner, McCann, was convicted for violating Selective Training and Service Act of 1940 after he waived his right to a jury trial without counsel present. He later argued that this waiver was not valid as it violated his constitutional rights under the Sixth Amendment which guarantees an accused person's right to a fair trial by an impartial jury. The Supreme Court held that while defendants can generally waive certain constitutional rights such as the right to counsel or protection against self-incrimination, they cannot waive their fundamental protections including their right to a public trial by an impartial jury unless done knowingly and intelligently with full understanding of implications involved. This decision emphasized that courts have an independent interest in ensuring trials are conducted within constitutionally established procedures even if defendants themselves may be willing to relinquish those safeguards.

In the dissenting opinion for UNITED STATES ex rel. MCCANN v. ADAMS, WARDEN, et al., Justice Frank Murphy argued that a defendant's right to counsel is so fundamental and absolute that it should not be waived without clear understanding of its implications. He believed that the majority decision failed to protect this essential constitutional safeguard adequately by allowing defendants to waive their right to counsel without fully comprehending what they were giving up. In his view, even if a defendant voluntarily chooses not to have legal representation, courts should still ensure they are making an informed decision with full knowledge of potential consequences and risks involved in self-representation during trial proceedings.