The United States v. Vowell and M'Clean was a Supreme Court case in which the defendants were charged with violating an act of Congress that prohibited trading with enemies during wartime. The defendants argued that they had not violated the law because they had traded goods to British subjects who resided in Spanish territory, rather than directly to Britain itself. However, Chief Justice John Marshall ruled against them, holding that it did not matter where the trade occurred; if it was done on behalf of Britain or its citizens then it constituted a violation of the act regardless of location. He also held that even though Spain may have been neutral in this conflict, any aid given to one side could be considered as aiding their enemy and thus illegal under American law. This ruling established an important precedent for future cases involving foreign commerce and neutrality laws during times of war.

In the case of United States v. Vowell and M'Clean, Chief Justice John Marshall delivered a dissenting opinion that argued against the majority's decision to acquit the defendants. He argued that Congress had not given any authority for an individual to be tried in a state court for treason committed against the United States, as it was solely within federal jurisdiction. Furthermore, he noted that if this precedent were allowed to stand then individuals could commit crimes with impunity since they would only have to flee from one state into another in order to avoid prosecution by federal authorities. Finally, he asserted that allowing such behavior would undermine public confidence in government and weaken its ability to protect citizens from harm or injustice.