The Plymouth was a Supreme Court case that dealt with the issue of whether or not an Indian tribe could be considered as a foreign nation. The court determined that, while the Indians were not citizens of any particular state, they did have certain rights and privileges under federal law. Specifically, the court held that Congress had exclusive authority to regulate commerce with Indian tribes and thus could protect them from being taken advantage of by other nations or individuals. Furthermore, it was found that treaties between the United States government and Indian tribes should be respected in order for those agreements to remain valid. Ultimately, this decision established important legal precedents regarding tribal sovereignty which still stand today.

In The Plymouth, the Supreme Court was tasked with determining whether a vessel seized by the United States during the Civil War could be forfeited for violating a blockade. Justice Field delivered an opinion dissenting from that of the majority. He argued that since there had been no declaration of war between the Union and Confederacy at this point in time, it would have been impossible to violate any blockade as such did not exist yet. Furthermore, he noted that even if there had been a valid blockade declared prior to seizure of The Plymouth, due process must still be followed before forfeiture can occur; namely, notice must be given to all parties involved so they may present their case before any decision is made on forfeiture or otherwise. As such, Justice Field concluded that without proper notification being provided beforehand regarding potential forfeiture proceedings against The Plymouth and its owners/captain(s), it was unconstitutional for them to suffer loss through confiscation by federal authorities without having first received fair warning about what might happen should they choose to sail into Confederate waters during wartime conditions.