At its peak, between the 12th and 13th centuries, Cahokia was the most populous city in North America. (Larger cities did exist in Mesoamerica and South America.) Monk's Mound, the major ceremonial center of Cahokia, remains the largest earthen construction of the prehistoric Americas. The culture reached its peak in about 1200–1400 CE, and in most places, it seems to have been in decline before the arrival of Europeans. Many Mississippian peoples were encountered by the expedition of Hernando de Soto in the 1540s, mostly with disastrous results for both sides. Unlike the Spanish expeditions in Mesoamerica, who conquered vast empires with relatively few men, the de Soto expedition wandered the American Southeast for four years, becoming more bedraggled, losing more men and equipment, and eventually arriving in Mexico as a fraction of its original size. The local people fared much worse though, as the fatalities of diseases introduced by the expedition devastated the populations and produced much social disruption. By the time Europeans returned a hundred years later, nearly all of the Mississippian groups had vanished, and vast swaths of their territory were virtually uninhabited. When the Europeans arrived, indigenous peoples of North America had a wide range of lifeways from sedentary, agrarian societies to semi-nomadic hunter-gatherer societies. Many formed new tribes or confederations in response to European colonization. These are often classified by cultural regions, loosely based on geography. These can include the following:  Numerous pre-Columbian societies were sedentary, such as the Pueblo peoples, Mandan, Hidatsa and others, and some established large settlements, even cities, such as Cahokia, in what is now Illinois. The Iroquois League of Nations or "People of the Long House" was a politically advanced, democratic society, which is thought by some historians to have influenced the United States Constitution, with the Senate passing a resolution to this effect in 1988. Other historians have contested this interpretation and believe the impact was minimal, or did not exist, pointing to numerous differences between the two systems and the ample precedents for the constitution in European political thought. Mesoamerica is the region extending from central Mexico south to the northwestern border of Costa Rica that gave rise to a group of stratified, culturally related agrarian civilizations spanning an approximately 3,000-year period before the visits to the Caribbean by Christopher Columbus. "Mesoamerican" is the adjective generally used to refer to that group of pre-Columbian cultures.