In Romero v. United States, the Supreme Court held that a person who had been convicted of an offense in one state could not be tried for the same offense in another state. The case arose when Francisco Romero was indicted by a grand jury in California on charges of larceny and burglary after he had already been convicted of those offenses in Nevada. The court found that under the Constitution's Full Faith and Credit Clause, states must give full faith and credit to each other's judicial proceedings, including criminal convictions. Therefore, since Romero had already been convicted once for his crimes, it would have violated this clause if he were subjected to double jeopardy by being tried again for them elsewhere. As such, the court ruled that his indictment should be quashed as unconstitutional.

In the case of Romero v. United States, Justice Nelson delivered a dissenting opinion in which he argued that Congress had no authority to pass laws concerning slavery and emancipation. He believed that only state legislatures could make such decisions, as they were the ones who created and regulated slavery within their own borders. Furthermore, he argued that any law passed by Congress would be unconstitutional because it would interfere with states' rights under the Tenth Amendment of the Constitution. Additionally, Nelson contended that if Congress was allowed to pass such legislation then it could potentially lead to further federal interference in other areas traditionally left up to individual states’ discretion. Ultimately, his dissent concluded by stating that while he did not condone or support slavery itself, allowing for Congressional action on this issue was an infringement upon state sovereignty and thus should not be permitted under our current system of government.