Why did epidemic diseases and colonial expansion push some Indigenous peoples westward onto the Plains?

The movement of Indigenous peoples onto the Great Plains was driven by the profound pressures of European colonization and the devastating impact of introduced epidemic diseases.

As Euro-American settlers expanded their territorial reach, particularly during the 19th century, they disenfranchised many Indigenous groups of their land and sovereignty through forced removal policies, such as the Indian Removal Act of 1830. This "land hunger" and the resulting violence forced many tribes to relocate westward to survive.12

Simultaneously, the introduction of European diseases—such as smallpox, measles, and others—caused catastrophic mortality rates, sometimes as high as 95 percent. This massive population loss destabilized traditional societies, forcing survivors to reorganize, adopt new roles, and in many cases, migrate to escape the reach of colonial conflicts and the depletion of their resources.341

For some groups, such as the Oceti Sakowin and Cheyenne, this westward shift involved a transition to a predominantly nomadic, equestrian way of life, which was facilitated by the horse and allowed them to adapt to the Plains environment.56