Estados Unidos Mexicanos is the successor State of Spain in the Northern region of the former Spanish empire in the Americas. Pope Alexander VI issued a papal bull on May 4, 1493, fixing the line of demarcation of conquered territories between the Kings of Spain and Portugal thereby avoiding future conflict; this papal bull in effect granted the then-unexplored territory of what is now Mexico to the Spanish crown. Heman de Cortes conquered the Aztec empire in 1521 and Spain ruled the territory until Mexican independence in the early 19th century. The territory was called Nueva Espana (New Spain) and its political capital was in what is today Mexico City.
Mexico in SOURCES OF STATE PRACTICE IN INTERNATIONAL LAW (2nd ed.) (Brill, 2014). Editors: Ralph Gaebler and Alison Shea. Librarian Scholarship Series. 36. http://digitalcommons.law.yale.edu/ylss/37