In United States v. Vallejo, the Supreme Court held that a Mexican land grant was valid and enforceable under California law. The case arose when the United States government attempted to invalidate a Mexican land grant given to General Mariano Guadalupe Vallejo in 1844 by Mexico's former governor of Alta California, Manuel Micheltorena. The US argued that it had acquired title to all lands within its boundaries as part of the Treaty of Guadalupe Hidalgo which ended the Mexican-American War in 1848 and thus any prior grants were void. However, the Supreme Court found that since no specific provision was made for such grants in either treaty or federal statute, they remained valid under state law and could not be revoked by Congress or other branches of government without due process protections being provided first. As such, General Vallejo retained his rights to his property despite changes in sovereignty over time.

In United States v. Vallejo, the Supreme Court was asked to decide whether a Mexican land grant could be recognized as valid in California after it had become part of the United States. The majority opinion held that such grants were not valid because they did not conform with American law and customs. Justice Field dissented from this decision, arguing that Congress had never explicitly declared these grants invalid and thus should be respected under the principle of comity between nations. He further argued that since Mexico had been willing to recognize similar claims by Americans before 1848, when California became part of the U.S., then there was no reason why those same rights should not continue to apply now that California is part of America's territory. In conclusion, he stated his belief that if Congress wanted to make any changes regarding these land grants it would have done so already instead of leaving them open for interpretation by courts or other government bodies without explicit direction from lawmakers themselves