Mexico, officially United Mexican States, Country, southern North America. The Rio Grande forms part of its northeastern border with the U.S. Mexico has two major peninsulas, the Yucatán in the southeast and Baja California in the northwest. The high Mexican Plateau forms the core of the country and is enclosed by mountain ranges the Sierra Madre Occidental, the Sierra Madre Oriental, and the Cordillera Neo-Volcánica. The last has the country’s highest peak, the volcano Citlaltépetl, which reaches 18,406 ft.