The Nassau was a case heard by the United States Supreme Court in 1866. The case involved a dispute between the owners of the Nassau, a British vessel, and the United States government. The owners of the Nassau had been accused of violating the Neutrality Act of 1794 by engaging in trade with the Confederate States of America during the Civil War. The owners of the Nassau argued that the Neutrality Act did not apply to them because they were British citizens and the vessel was registered in the United Kingdom. The Supreme Court ruled in favor of the United States government, finding that the Neutrality Act did apply to the owners of the Nassau. The Court held that the Neutrality Act was a valid exercise of the United States' power to regulate foreign commerce and that the Act applied to all vessels, regardless of their nationality. The Court also held that the United States had the right to seize and confiscate the Nassau and its cargo as a penalty for violating the Neutrality Act. The Court's decision established the principle that the United States could regulate foreign commerce and punish those who violated its laws, even if they were foreign citizens.

In The Nassau, the Supreme Court was asked to decide whether a contract between two parties that had been made in violation of a federal statute could be enforced. In an 8-1 decision, the court held that it could not. Justice Field delivered the dissenting opinion and argued that while he agreed with the majority's conclusion on its face value, he believed there were other considerations at play which should have led to a different outcome. He noted that if contracts are invalidated simply because they violate federal statutes then this would lead to uncertainty and confusion among those entering into agreements as well as create difficulties for courts trying to interpret them. Furthermore, Field argued that such an interpretation would also undermine public policy by discouraging individuals from engaging in lawful activities due to fear of having their contracts declared voidable or unenforceable. Ultimately, Field concluded by asserting his belief that when considering cases like these where both parties acted in good faith and without knowledge of any violations of law or fraud involved then justice should prevail over technicalities alone.