The United States v. Vanzandt was a Supreme Court case in which the defendant, John Vanzandt, had been convicted of violating an act of Congress that prohibited trading with Native Americans without permission from the government. The court held that although there were no specific words in the statute prohibiting such trade, it was implied by its general language and purpose. Furthermore, they found that even if this interpretation could be considered too broad or vague to be enforced as law, it would still stand since Congress had passed it with full knowledge of what they intended for it to mean. Ultimately, the court ruled against Vanzandt and upheld his conviction under this act.

In the case of The United States v. Vanzandt, the Supreme Court was tasked with determining whether a defendant could be convicted for an act that had been criminalized after it was committed. In this particular case, Congress had passed a law making it illegal to bring goods into the country without paying duties on them before they were imported. The defendant argued that he should not be held liable since his actions occurred prior to when Congress made such acts illegal and thus there was no way for him to have known about or followed the new laws at that time. However, in a 5-4 decision, the majority opinion found against him and stated that ignorance of existing laws is no excuse; therefore he must still face conviction as if he had broken those laws at their inception even though they did not exist yet when his offense took place.