Final answer:
The settlement of the Great Plains was one of the main causes of the Indian Wars between 1860 and 1890.
Step-by-step explanation:
One of the MAIN causes of the Indian Wars that occurred between 1860 and 1890 was the settlement of the Great Plains. As the US government encouraged westward expansion, settlers and ranchers moved onto the Great Plains, encroaching upon the traditional lands of Native American tribes. This led to conflicts over land and resources, resulting in the Indian Wars.
Learn more about Causes of the Indian Wars