In United States v. Sepulveda, the Supreme Court of the United States held that a person who was born in Mexico and had resided there for many years could not be considered an American citizen without proof of naturalization or other evidence showing his allegiance to the United States. The case arose when Juan Sepulveda was arrested by federal authorities on charges of treason against the United States during the Civil War. He argued that he should not have been charged with treason because he was an American citizen, but this claim was rejected by both lower courts and ultimately by the Supreme Court as well. The court reasoned that since Sepulveda had never taken any steps to become a naturalized U.S. citizen, he could not be considered one under existing law at that time and thus did not enjoy all rights associated with citizenship such as protection from being charged with treason against his own country

In United States v. Sepulveda, the Supreme Court was asked to decide whether a person born in Puerto Rico before it became part of the United States could be considered an American citizen. The majority opinion held that such persons were not citizens and thus did not have access to certain rights under the Constitution. In his dissenting opinion, Justice Field argued that Congress had intended for all people living in territories acquired by the U.S., including those who were already there when they were acquired, to become citizens upon acquisition of those territories. He further noted that even if this was not Congress' intent at first, their subsequent actions showed clear recognition of citizenship for these individuals as evidenced by various laws passed granting them certain privileges and immunities associated with being a citizen of the United States. Therefore he concluded that any other interpretation would lead to unjust results and should be rejected in favor of one which recognized citizenship for persons born in Puerto Rico prior to its acquisition by America