Armstrong v. United States was a Supreme Court case decided in 1871. The case involved a dispute between the United States and the estate of a deceased soldier, William Armstrong. Armstrong had served in the Union Army during the Civil War and had been promised a pension for his service. After his death, his widow and children applied for the pension, but the United States refused to pay it, arguing that Armstrong had not been properly discharged from the Army. The Supreme Court ruled in favor of the Armstrong family, finding that the United States had failed to provide sufficient evidence that Armstrong had not been properly discharged. The Court held that the United States was obligated to pay the pension, as it had promised to do so in exchange for Armstrong's service. The Court also noted that the United States had failed to provide any evidence that Armstrong had not been properly discharged, and thus the burden of proof was on the United States. The decision in Armstrong v. United States established an important precedent in the area of veterans' benefits. It established that the United States was obligated to honor its promises to veterans, and that it had the burden of proof in cases involving veterans' benefits. The decision also established that veterans and their families were entitled to receive the benefits they had been promised.

In the case of Armstrong v. United States, Justice Field delivered a dissenting opinion in which he argued that the majority's decision was contrary to established precedent and would lead to an unjust result. He noted that prior cases had held that when Congress passed legislation with respect to Indian tribes, it should be interpreted liberally in favor of those tribes so as not to deprive them of their rights without clear language indicating such intent. In this case, however, the Court had adopted a narrow interpretation of the statute at issue and thus denied relief for Armstrong despite his having acted in good faith reliance on earlier decisions from lower courts granting him permission to settle on tribal lands. Justice Field concluded by noting that while Congress may have intended for its laws regarding Indian land settlements to be strictly enforced, it did not intend for individuals who relied upon existing judicial interpretations thereof—such as Armstrong—to suffer injustice because they were later overturned by higher courts.