Alviso v. United States was a Supreme Court case that addressed the issue of whether a Mexican citizen who had been living in California since before the Mexican-American War was entitled to the same rights as a naturalized citizen. The case was brought by Francisco Alviso, a Mexican citizen who had been living in California since 1841. Alviso argued that he was entitled to the same rights as a naturalized citizen, including the right to own land. The Supreme Court ruled in favor of Alviso, holding that he was entitled to the same rights as a naturalized citizen. The Court reasoned that Alviso had been living in California since before the Mexican-American War, and thus had been living in the United States for a sufficient amount of time to be considered a citizen. The Court also noted that Alviso had been living in California in good faith, and had not taken any action to renounce his citizenship. The Court's ruling in Alviso v. United States was significant because it established that Mexican citizens who had been living in the United States prior to the Mexican-American War were entitled to the same rights as naturalized citizens. This ruling was an important step in recognizing the rights of Mexican citizens living in the United States.

In the case of Alviso v. United States, the Supreme Court was asked to decide whether a Mexican citizen who had been living in California since before it became part of the United States could be considered an American citizen. The majority opinion held that he could not because he had not gone through any naturalization process and thus did not meet all of the requirements for citizenship under U.S. law at that time. However, Justice Field dissented from this decision on two grounds: first, that Congress has no power to deny citizenship to someone born in a territory acquired by treaty; and second, even if they do have such power, it should only apply prospectively rather than retroactively as would be required here given Alviso's long-standing residence in California prior to its acquisition by treaty with Mexico. He argued further that denying him his rights as an American citizen would violate both due process and equal protection principles enshrined within our Constitution regardless of when those rights were granted or denied