The Nichols v. United States case was a Supreme Court case that dealt with the issue of whether a person could be convicted of a crime if the act was not illegal at the time it was committed. The case involved a man named William Nichols, who was convicted of violating the Neutrality Act of 1794. The act made it illegal to aid or assist a foreign nation in a war against the United States. Nichols had been accused of providing supplies to the Confederate States during the Civil War. At the time of Nichols' actions, the Neutrality Act of 1794 was still in effect. However, the Supreme Court ruled that the act was no longer applicable because the Civil War had changed the political landscape of the United States. The Court held that the act was no longer applicable because it was not intended to apply to a civil war between two parts of the same nation. Therefore, the Court ruled that Nichols could not be convicted of violating the Neutrality Act of 1794.

In the case of The Nichols, a majority opinion was reached that held that an act passed by Congress in 1864 allowing for the sale of certain public lands to satisfy pre-existing debts did not apply retroactively. However, Justice Field dissented from this ruling and argued that it should be applied retroactively. He reasoned that since the law had been enacted before any payments were made on these debts, it could be assumed to have been intended to cover them as well. Furthermore, he noted how applying such laws only prospectively would create uncertainty and confusion among those who may have relied upon its provisions when making decisions about their financial affairs prior to its passage. Ultimately, Justice Field concluded his dissent by arguing against what he saw as a lack of fairness in denying debtors relief under this law due to circumstances beyond their control.