In The United States vs. Vallejo, the Supreme Court was asked to decide whether a Mexican land grant in California could be legally recognized by the U.S. government after it had been granted before California became part of the United States following its annexation from Mexico in 1848. The court held that although Congress had not specifically authorized such grants, they were valid and enforceable under existing laws because they were made prior to statehood and did not conflict with any federal law or policy at that time. Furthermore, since these grants had already been accepted as valid by both parties involved when California joined the Union, it would be unfair for one party to now challenge their validity without good cause or legal justification. Therefore, this decision established an important precedent for recognizing pre-statehood land grants throughout America's western territories and states acquired through treaty or conquest from other nations during this period of expansionism in American history.

In United States v. Vallejo, the Supreme Court held that a Mexican land grant was not valid because it had been made by an authority without legal power to do so. The dissenting opinion argued that the court should have recognized the validity of such grants as long as they were accepted and acted upon in good faith by both parties involved. The dissent further noted that Congress had already passed legislation recognizing certain Mexican land grants, and thus it would be inconsistent for the court to refuse to recognize them now. Furthermore, since many settlers relied on these grants for their livelihoods, denying them recognition could cause significant hardship and injustice if those settlers were forced off their lands due to lack of title or other legal protection. Ultimately, while acknowledging some potential issues with allowing such claims based solely on acceptance in good faith rather than formal legality under Spanish law at the time of acquisition, this opinion concluded that justice demanded recognition of these rights nonetheless given all relevant circumstances surrounding each case individually.