The Greenwood v. United States case in 1955 revolved around the issue of whether a defendant, who was found mentally incompetent to stand trial, could be committed indefinitely without a jury trial. The petitioner, Greenwood, had been indicted for bank robbery but was declared unfit to stand trial due to his mental condition. He was then committed by the court into custody until he became sane enough for prosecution under Section 4246 of Title 18 U.S.C., which allows such commitment without a jury's decision on insanity. Greenwood argued that this violated his constitutional rights as it effectively allowed indefinite detention based on judicial discretion alone and not through due process involving a jury verdict about his sanity or dangerousness. However, the Supreme Court ruled against him stating that Section 4246 did not violate any constitutional rights because it only applied when an accused person is already before the court facing charges and has been determined unable to understand proceedings or assist in their own defense. Furthermore, they clarified that such commitment is not punitive but rather protective - both for society from potentially dangerous individuals and also for defendants themselves who may need treatment.

In the dissenting opinion for Greenwood v. United States, it was argued that the court's decision to allow a person to be committed indefinitely without a jury trial violated their constitutional rights. The justice believed that this ruling went against both the Fifth and Sixth Amendments, which guarantee due process of law and the right to a speedy public trial by an impartial jury respectively. They contended that these protections should extend even in cases where mental illness is involved, as they do not believe insanity negates one's basic human rights or legal protections under U.S law. Furthermore, they expressed concern over potential abuses of power if individuals could be involuntarily confined based on subjective psychiatric evaluations without any form of judicial review or oversight from a jury.