In the 1918 case Brothers v. United States, the Supreme Court examined whether a defendant could be convicted of conspiracy to defraud the U.S. government if they were unaware that their actions were illegal. The defendants in this case had been involved in a scheme to sell liquor on an Indian reservation, which was prohibited by federal law at that time. They argued that they did not know selling alcohol on reservations was illegal and therefore could not have intended to defraud the government. The Supreme Court ruled against them, stating ignorance of the law is no excuse for breaking it - even when it comes to complex laws about fraud or conspiracy. The court held that as long as there's evidence showing defendants agreed together knowingly to commit an act which is unlawful (in this instance, selling liquor), then they can be found guilty of conspiracy regardless of whether or not they knew their agreement would result in fraud against the U.S.

In the dissenting opinion for Brothers v. United States, the justice argued that the majority's decision was inconsistent with previous rulings and interpretations of federal law. The justice disagreed with the majority's interpretation of what constitutes a "sale" under section 2 of the Harrison Narcotic Act, arguing that it should not include giving away drugs as part of a medical treatment plan by physicians to their patients who are drug addicts. He contended that such an interpretation would criminalize legitimate medical practices and infringe upon states' rights to regulate medicine within their borders. Furthermore, he believed this ruling could potentially lead to unjust prosecutions against doctors acting in good faith while treating patients suffering from addiction.