The United States Supreme Court case of The United States v. Francisco Pico and Others involved a dispute between the U.S. government and several Mexican citizens over land in California that had been granted to them by Mexico prior to the 1848 Treaty of Guadalupe Hidalgo, which ended the Mexican-American War and ceded much of what is now known as the American Southwest from Mexico to the U.S.. The court ruled that while it was true that these individuals were legally entitled to their lands under Mexican law, they could not be recognized as such under American law because they did not possess any valid title documents or other proof of ownership when California became part of America following its acquisition through treaty with Mexico in 1848; thus, all claims for private property rights must be based on actual possession at some point after this date if those rights are going to be recognized by an American court.

In the case of United States v. Francisco Pico and Others, the Supreme Court was tasked with determining whether a Mexican land grant issued to Pico by Mexico in 1845 was valid under U.S. law after California became part of the United States following its annexation in 1850. The majority opinion held that since it had been granted before California's annexation, it could not be recognized as valid under U.S law because any grants made by foreign governments are invalid when they conflict with laws passed by Congress or state legislatures within their jurisdiction; however, Justice Grier dissented from this opinion on two grounds: firstly, he argued that there were no laws passed at the time which would have rendered such grants invalid and secondly he maintained that even if there were such laws they should not apply retroactively to pre-existing grants like Pico's which had already vested rights upon him prior to their enactment. He concluded his dissent stating "It is an established principle of public law...that a grant once legally made cannot be recalled."