In United States vs. Covilland et al., the Supreme Court ruled on a case involving an alleged violation of the Neutrality Act of 1794, which prohibited citizens from engaging in acts that could be seen as aiding or assisting foreign nations at war with each other. The defendants were accused of attempting to purchase arms and ammunition for use by one such nation, Mexico, against another nation, Spain. The court found that while there was evidence suggesting that some individuals had attempted to acquire weapons for Mexico's use against Spain, it did not constitute a violation of the Neutrality Act because no actual transaction took place between any parties involved in this matter and thus no aid was provided to either country. Furthermore, even if such an act had taken place it would have been considered lawful under international law since both countries were engaged in hostilities at the time. Ultimately then, the court held that none of those charged with violating neutrality laws could be convicted due to lack of sufficient proof beyond reasonable doubt regarding their involvement in any illegal activity related to this case.

In United States vs. Covilland et al., the Supreme Court was asked to decide whether a federal statute that allowed for the seizure of vessels engaged in illegal slave trading applied retroactively. The majority opinion held that it did, but Justice Grier dissented on this point and argued that Congress had no power to pass such a law with retrospective effect. He reasoned that if Congress could do so, then they would be able to punish people for acts committed before any laws were passed prohibiting them - something he believed was unconstitutional as it violated due process rights and constituted an ex post facto law. Furthermore, he noted how there is nothing in the Constitution which grants Congress authority over foreign commerce or slavery; thus, any attempt by them to regulate these matters through legislation should not be given retroactive effect since doing so would exceed their powers under the Constitution.