Garnett v. United States was a Supreme Court case that addressed the issue of whether a person could be convicted of treason without being tried by a jury. The case involved a man named William Garnett, who was accused of treason against the United States for aiding Confederate forces during the Civil War. Garnett argued that he could not be convicted of treason without a jury trial, as the Sixth Amendment of the Constitution guarantees the right to a jury trial in criminal cases. The government argued that the Sixth Amendment did not apply to treason cases, as the Constitution does not explicitly mention jury trials in treason cases. The Supreme Court ultimately sided with Garnett, ruling that the Sixth Amendment does apply to treason cases. The Court reasoned that the right to a jury trial is a fundamental right that should be extended to all criminal cases, including treason. The Court also noted that the Constitution does not explicitly exclude treason cases from the right to a jury trial, and that the right to a jury trial is essential to ensure a fair trial. In its ruling, the Supreme Court established that the right to a jury trial applies to all criminal cases, including treason. This ruling has been cited in numerous subsequent cases, and has been an important precedent in the protection of the right to a jury trial.

In the case of Garnett v. United States, the Supreme Court was asked to decide whether a defendant who had been convicted in a state court and sentenced to death could be tried again for the same crime in federal court. The majority opinion held that such double jeopardy did not apply because there were two separate sovereigns involved—the State and Federal governments—and thus each government had its own right to prosecute an individual for crimes committed within their respective jurisdictions. However, Justice Field dissented from this decision arguing that it violated both common law principles as well as constitutional protections against double jeopardy found in the Fifth Amendment which states “nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb”. He argued that since these rights are fundamental they should not depend on which government is prosecuting an individual but rather should remain consistent regardless of jurisdiction or sovereignty.