In the 1941 case of Glasser v. United States, the U.S. Supreme Court ruled that a defendant's right to effective counsel was violated when his lawyer was appointed by the court to simultaneously represent another co-defendant with conflicting interests. The case involved two defendants, Glasser and Kretske, who were convicted for conspiracy to defraud the United States under federal narcotics laws. Both were represented by the same attorney during their trial despite objections from Glasser about potential conflicts of interest due to differing defense strategies between him and Kretske. In its decision, the Supreme Court emphasized that an accused has a constitutional right under Sixth Amendment not only to assistance of counsel but also "to have such assistance as will guarantee him 'the aid of counsel acting in his sole behalf.'" The ruling set important precedent regarding legal representation rights in criminal cases.

In the dissenting opinion for Glasser v. United States, Justice Frank Murphy argued that there was a clear conflict of interest in having one attorney represent both defendants, which violated their Sixth Amendment rights to effective counsel. He believed that this dual representation prevented the attorney from fully advocating for each defendant's best interests and potentially influenced his strategic decisions during trial. Furthermore, he disagreed with the majority's view that Glasser had implicitly waived his right to separate counsel by not objecting earlier in proceedings; instead, he asserted that such a waiver should be explicit and informed. Ultimately, Justice Murphy contended that upholding these convictions would set a dangerous precedent undermining defendants' constitutional rights.