Mexico is defined by dramatic geographic features that have shaped the country's politics. Forming the southern portion of North America, Mexico borders the tropical Central American isthmus, a broad band that defines Mexico's northern border with the United States. To the east and west, the Sierra Madre Occidental and the Sierra Madre Oriental dominate the Pacific and the Caribbean coasts, forming highlands that cradle of the modern heart of Mexico. The desolate Baja California peninsula shields approaches from the western sea. To the east, the tropical limestone outcropping named the Yucatán peninsula dominates entry from the Caribbean into the Gulf of Mexico. Mexico's central valley, which includes Mexico City and the adjacent Veracruz region, form Mexico's core. To control Mexico City is to control Mexico, and the greatest conventional threats to Mexico City have traditionally come from the deepwater Port city of Veracruz. Since the 16th century, Spain, France and the United States — not to mention numerous mutinous factions of the Mexican military — have used Veracruz to threaten and even conquer what is now Mexico City. Mexico's mountains and frontier territories are traditional hotbeds of unrest. From the rebels of Chihuahua to the drug kingpins of Tierra Caliente and Sinaloa, the historical and ongoing challenge for Mexico is to subdue and incorporate far-flung, geographically-isolated communities. The modern expression of this struggle can be seen in today's war with drug trafficking organizations, but it is a pattern that has been repeated throughout history and has been shaped by Mexico's physical geography.