borders and frontiers: The Americas 131 topographic limiters or barriers (mountains, rivers, forests, and so on); competition for space, resources, or power; the relationship of a dependent periphery to a central core; trade; tribute; migration; warring; and, on a broad scale, climate change New technologies allowed for expansion into previously uninhabitable areas, thereby shifting, expanding, or challenging both borders and frontiers Any combination of these factors contributed to the classification and continuous redefinition of borders throughout the Americas Typically, however, the literature has had more to say about cultural and symbolic boundaries between groups than about the concrete, physical borders between them Broadly speaking, because written documentation is unavailable, the exact nature of the political reality of borders is unclear, but it is certain that people understood the notion of territory as a thing to be defended or taken The North American landscape during the thousandyear period 500 to 1500 comprised vast swaths of variable terrain traversed by hunter-gatherer bands, entire regions such as the Arctic and the Pacific Northwest inhabited by semisedentary groups moving between summer hunting and fishing grounds and winter residences, and established agrarian communities in the southeastern and southwestern regions that depended on advanced irrigation techniques or plentiful hunting and fishing to maintain settled communities Various models have been suggested for understanding the relationships of peoples in the Southwest, including the Hohokam, Anasazi, and Mogollon in the period 500 to 1400 “Regional” and “macroregional” systems propose interaction spheres tied together by long-distance exchange routes While they were clearly interacting, relationships were not always peaceful The extraordinary physical locations of the 14th-century Anasazi cliff dwellings at Mesa Verde and the later Hopi mesa-top settlements are clearly defensive minded and indicate regular border infractions by raiding and warring parties Border definition in the southeastern areas between the 10th and 14th centuries is equally vague Similar to the earlier Midwest-centered Hopewell culture (200 b.c.e.–400 c.e.) “sphere of influence,” the southeastern region is often described as an “interaction sphere” or “interaction network” within which large and small independent polities existed The largest of the Mississippian centers, Cahokia, settled near the confluence of the Missouri and Mississippi rivers With an estimated population reaching 40,000, Cahokians directly controlled a large territory and had strong influence on groups a thousand miles away, perhaps as far as the Iroquois region in the Northeast, though direct administrative control was considerably less Similarly, the Mesoamerican situation varied greatly over time The dissolution of the central Mexican empire centered at Teotihuacán by the eighth century created a vacuum and melted previous affiliations and borders Numerous Mayan polities existed independently, though extensive trade ties and even military affiliations were common The late rise of the Aztec Empire in the 13th century reconsolidated disparate cultures and polities under a single authoritarian rule with genuinely adhesive borders In the Late Classic Period (650–900) Mayan power polities were divided among an increased number of centers, which suggests that the period was less centralized economically and politically Tikal in the northern Guatemalan jungles of the Petén, Yaxchilan in the Usumacinta region, Palenque in the southwestern region of Chiapas, and Copán in the southeast just over the Honduran border with Guatemala were all major centers that existed independently, each with various spheres of influence and border affinities Some evidence suggests that Yaxchilan was a military power that led an alliance of several primary centers, including possibly Tikal and Palenque Increased fortification indicates antagonistic relations that challenged the sovereignty of borders Tikal occupied a strategic military position that offered natural defensive measures that effectively safeguarded it against attack Swamps to the east and west greatly limited threats from those directions The north and south were defended by a system of man-made earthworks, consisting of a shallow moat and an interior rubble wall, which may have been constructed during the Protoclassic Period (100–250) or early Classic Period (ca 250–550) and used throughout Tikal’s ascendancy until about 900 Trade contacts established earlier with Teotihuacán, the central Mexican empire from roughly the first to the seventh centuries, helped solidify its borders and frontiers Similarly, the Mayan site of Becán showed defensive motives Situated in the heart of the Yucatán Peninsula roughly 150 miles north of Tikal, Becán was named after its most distinctive feature, an encircling moat and rampart (from becán, meaning “ditch filled with water”) Excavations revealed that the moat was originally some 16 feet deep and more than 52 feet wide, with its interior rampart rising another 16 feet Ceramic evidence suggests Becán was settled around 550 in an area with promising agricultural potential as well as control of local trade routes Subsequent fluctuations in population indicate shifting fortunes, and defensive facilities were built to maintain Becán’s political and economic control over the region The central Mexican Aztec Empire founded at what is today Mexico City rose to prominence throughout the 14th and 15th centuries by overrunning one nearby city after the next, greatly extending their borders and eventually establishing a tribute empire encapsulating a wide swath of central Mexico