Nichols v. United States was a Supreme Court case that addressed the issue of whether a person could be convicted of treason against the United States if they had not committed any overt act of treason. The case involved William H. Nichols, who was accused of conspiring to levy war against the United States. The Supreme Court held that a person could be convicted of treason without committing an overt act of treason, as long as there was evidence of an agreement to commit treason. The Court reasoned that the Constitution's definition of treason was broad enough to include a conspiracy to levy war against the United States. The Court also noted that the Framers of the Constitution had intended to make it difficult to commit treason, and that requiring an overt act of treason would make it too easy to escape conviction. The Court concluded that a person could be convicted of treason without committing an overt act of treason, as long as there was evidence of an agreement to commit treason. In the end, the Supreme Court affirmed the conviction of William H. Nichols for treason. This case established the precedent that a person can be convicted of treason without committing an overt act of treason, as long as there is evidence of an agreement to commit treason.

In the case of Nichols v. United States, the Supreme Court was asked to decide whether a person who had been convicted in a state court for an offense that would have been punishable by death if committed within federal jurisdiction could be sentenced to more than six years in prison under federal law. The majority opinion held that such persons could not receive sentences longer than six years, as this would constitute double jeopardy and violate the Fifth Amendment's prohibition against being tried twice for the same crime. However, Justice Field dissented from this decision on two grounds: firstly, he argued that Congress had intended to allow longer sentences when it passed legislation allowing states to prosecute offenses which were also punishable by death under federal law; secondly, he argued that even if Congress did not intend such a result at the time of passing said legislation, it should now be interpreted as permitting longer sentences due to changed circumstances since then.