Before European arrival, the region that would become Illinois Country was home to the biggest Native city-state on the continent, Cahokia. Numbering twenty thousand inhabitants at its height, Cahokia shaped the trade and culture of peoples in a huge portion of the continent. Although the story of Cahokia is well told by historians, it is often treated in isolation, disconnected from later historical events. For our purposes, we need to view it as part of a long-term set of processes that continued long after Cahokia was no more.⁹ The Illinois were not descendants of the Cahokians, but the Illinois’s rise in the Illinois Valley can be seen as a consequence of some of the same forces that brought Cahokia to an end.

      Cahokia rose in a region at the confluence of the Missouri, Mississippi, Illinois, and Ohio rivers, the “American Bottom.”¹⁰ Based heavily on corn agriculture, the civilization spread its power throughout the Midwest, as revealed by archaeological evidence of trade and tribute coming into the metropolis from the eleventh through the thirteenth century from throughout an expansive territory.¹¹ Beginning as an ordinary village in the Late Woodland period, Cahokia experienced a “big bang”—a sudden and dramatic rise to power.¹² By the 1200s, the twenty thousand people in Cahokia represented the largest pre-Columbian city north of Mexico. It is not clear whether Cahokia was a truly centralized political regime, but the Cahokia Mississippians exercised wide regional influence through a hegemonic culture and trade.¹³

      The middle of the continent was a likely place for the most powerful pre-contact Native society for the same reason that Chicago and St. Louis rose in the nineteenth century: the Midwest contained a tremendously rich variety of ecological resources.¹⁴ Given the major river systems that defined the region, the landscape contained numerous alluvial environments with resources for human exploitation. These environments featured wetlands and forests, often with extensive floodplains. Above the river valleys were a mix of hardwood forests, dominated mostly by oak-ash-maple and oak-hickory forests. These forests transitioned into park-like edge habitats, probably maintained by purposeful burning and natural fire, which in turn gave way to plains and the Midwest’s distinctive tallgrass prairies. A discrete biome unto itself, the tallgrass prairie was where evaporation and rainfall were roughly equal but where trees could not establish themselves because of periodic fire, dense grass roots, and other factors.¹⁵ While 39 percent of the modern state of Illinois was forested before the advent of the steel plow, fully 55 percent of the state’s landscape was covered with prairie. Much of the rest was dominated by wetlands.¹⁶

      Despite all the diversity of the region, the Cahokians took advantage of a relatively small portion of the ecological opportunity in the middle of the continent. Because environmental conditions were so favorable for farming, their subsistence focused on a small area, near the confluence of the rivers, and was heavily concentrated in the bottomlands.¹⁷ This proved perilous. Climate change started to affect the region in the 1100s. For 140 out of 145 years beginning in 1100, Cahokia experienced drought, probably reducing farming yields.¹⁸ Sediment in the Missouri River owing to drought-forced erosion on the Plains may have produced a shallower channel in the Missouri and Mississippi rivers and increased spring flooding in the bottomlands, compromising agriculture.¹⁹ Meanwhile, even as drought challenged their subsistence, it seems clear that Cahokians overexploited resources, especially wood, in their local environment, which may have led to difficulties in the city.²⁰ In the 1250s, a new Pacific climate event began to change conditions again, bringing cooler summers and harsher winters, and drier conditions overall.²¹ With the onset of the Pacific episode, farming became even more difficult at Cahokia. As a result of this and probably other social and political changes, the entire region began to empty out, leaving the middle Mississippi Valley, the lower Illinois Valley, the lower Ohio Valley, and the entire American Bottom increasingly devoid of people. The emerging “vacant quarter” was a dramatic end to the great civilization at Cahokia.²²

      With the end of the Cahokia, smaller groups reoccupied the region of Illinois. Most likely these migrants came from the West as climate change extended the so-called prairie peninsula and made conditions on the Plains drier and colder. Seeking refuge for a mixed-subsistence lifeway in the river valleys of the Illinois, these people were the Oneota, the ancestors of later Siouan-speakers like the Winnebago, Otoes, Ioways, and others.²³ Compared to Cahokia, they lived on a smaller scale and took advantage of a much larger variety of the ecology of the region.²⁴ In contrast to the complexity of Cahokians, they were “simple” and regularly relocated villages to take advantage of different aspects of the local ecology.

      Although we cannot be precise about details, archaeological evidence suggests that the Oneota newcomers were factionalized and organized into small social units.²⁵ They also lived a rather Hobbesian existence, since violence was endemic in the Illinois after Cahokia’s decline. Archaeological sites from the 1300s and 1400s in the Illinois Valley reveal fortified villages and other evidence of warfare between Oneota and Middle Mississippian groups. Perhaps the most dramatic evidence of violent lifeways in this period is an Oneota village, dated to the year 1300, in the central Illinois Valley. The site contains a cemetery with 264 burials. Sixty-six percent of the people interred at the cemetery had been decapitated or scalped at death.²⁶ Life was hard in the wake of Cahokia.

      In addition to warfare, other factors kept the population of the Oneota villagers low. Continuing the trend of the Pacific event that started in the 1200s, another climate episode began in the 1400s: the Little Ice Age. While this created well-known disturbances around the world and especially in Eurasia, it also brought about a significant shift on the North American continent.²⁷ By some estimates, the Little Ice Age may have reduced the average summer temperatures by 1.5 degrees Celsius in mid-America.²⁸ The temperature shifts may have produced as many as thirty-four fewer frost-free days in the modern state of Illinois. The continuing low human population in the vacant quarter that this and previous climate events helped occasion may be responsible for producing a rather sudden and dramatic species shift in the region. With few people in the Illinois Valley, and thus so few potential hunters, the population of wild animals may well have spiked.²⁹ By the 1500s, the great wild ungulates of North America, bison, had invaded the grasslands east of the Mississippi River in large herds, probably extending the prairies with their grazing as they advanced.³⁰ They came in great numbers, congregating by salt licks and springs, such as near Starved Rock in the Illinois Valley.³¹

      There is an important reason why bison revolutionized lifeways for the Oneota occupants of the midwestern prairies. As is well-known, tallgrass prairies in the middle of the country sat atop the thickest topsoil layer anywhere in the world.³² Yet the