U.S. citizenship is a legal status that grants individuals rights and privileges under the Constitution and laws of the United States, including the right to vote, work, and reside in the country. This status became particularly significant during the 19th century as the U.S. expanded its territories, especially following the Mexican-American War, which led to new legal and social dynamics in areas like New Mexico. The acquisition of territories and the complexities of citizenship played a critical role in shaping the identity and governance of these regions.