Ward v. United States was a Supreme Court case that addressed 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 Ward who was charged with the crime of selling liquor to Native Americans in violation of a federal law. Ward argued that the law was not in effect at the time he committed the act, and thus he could not be convicted of the crime. The Supreme Court ultimately sided with Ward, ruling that a person cannot be convicted of a crime if the act was not illegal at the time it was committed. The Court reasoned that a person should not be held responsible for something that was not a crime when it was done. The Court also noted that the law in question was not retroactive, meaning it did not apply to acts committed before it was passed. The decision in Ward v. United States established an important legal principle that a person cannot be convicted of a crime if the act was not illegal at the time it was committed. This principle has been applied in numerous cases since then, and it serves as an important protection for individuals who may be accused of a crime for something that was not illegal at the time it was done.

In Ward v. United States, the Supreme Court was asked to decide whether a federal statute that prohibited the sale of liquor within four miles of an Indian reservation violated the Fifth Amendment's protection against taking private property for public use without just compensation. The majority opinion held that Congress had not taken any private property and thus no violation had occurred. However, in his dissenting opinion Justice Field argued that although Congress did not take possession or title to any land when it passed this law, it still effectively took away valuable rights from landowners by prohibiting them from selling liquor on their own land near Indian reservations. He further argued that these restrictions were tantamount to a taking since they deprived owners of all economic value associated with their lands and therefore should be subject to just compensation under the Fifth Amendment.