How far into North America did the diplomatic/economic sphere of the Aztecs and other Mesoamerican empires extend? For instance, would a Native American living on the Chesapeake have heard of massive, city-building empires to the south?

Connections between what is now the American Southwest and Mesoamerican are so well established that it is almost inappropriate to think of them are entirely separate areas. These links were primarily trade based, not political or military, with products like tropical birds/feathers and cacao, as well various other products like cotton, rubber, and copper goods flowing north from Mesoamerica in exchange for Southwestern goods, primarily turquoise (Phil Weigand's work is focused on this). There were cultural links as well, with Mesoamerican style ballcourts appearing in the Southwest and an endless discussion of whether symbolic elements of Southwest and Mesoamerican religions were borrowed, adopted, or simply convergent. McGuire's (1980) paper is dated, but represents not only a seminal summation of the evidence for connections, but also a turning point in moving away from the older diffusionist idea that the complex societies in the Southwest were necessarily founded by long distance traders from Mesoamerica.

The Meso-SW connect, running up through West Mexico, reached its peak in the early (Mesoaemrican) Postclassic, about 900-1200 CE. This is pre-Aztec, after the decline of Teotihuacan and the Classic Maya, during the time when the Toltecs in Central Mexico and Chaco Canyon in the Southwest. There several sites in NW Mexico/SW US which flourished during this period as a result of this connect, such as Alta Vista and La Quemada. After this period, the Southwest and Mesoamerica see a period of drought and aridity (ironically known better as the Medieval Warm Period) which would lead to a decline in these interstitial groups. This climatic change was also a factor in the dissolution of the dense, complex societies in the SW and would spur the migration of Nahua groups from the Chichimec region into the Basin of Mexico, where they would eventually found the Aztec state.

Outside the Southwest, however, there is little indication of contact between Mesoamerica and other North American groups. Indeed, the only Mesoamerican linked artifact found outside the SW-Meso context is a single obsidian scaper. The inevitable question is "why not more connection?" but "why" is rarely a useful question in examining historical trends. One thing to take into account is the immense distances involved and the lack of pack animals. Direct trade between Tula and Cahokia, for instance, would walking from central Mexico to Missouri. Moreover, because humans can't graze like pack animals, it would have involved feeding all the porters carrying goods, which meant they would have had to carry supplies for themselves across long distances with uncertain resupply. Hassig in Aztec Warfare makes the case that these sort of logistic limitations were key in Mesoamerica being a hegemonic political system, rather than one that routinely exercised direct control. It's a useful concept to consider with regards to long distance trade as well.