The Plains Indians certainly have a very long and involved history. Archeological evidence suggests that Native Americans inhabited the Plains of Wyoming since 8500 B.C. Other archeological evidence indicates that Native Americans have lived in the Plains of Texas for 38,000 years. The arrival of the Europeans in the 1500s brought significant and sometimes devastating consequences. Along with trade, Europeans brought with them diseases Native Americans had not yet built an immunity to, including smallpox, measles, whooping cough, and others--all of which wiped out many populations of Native Americans. However, despite bringing illnesses, Europeans also brought horses and firearms to trade (Encyclopedia of the Great Plains, "Native Americans").
Though it was illegal for Spanish immigrants to trade with Native Americans using firearms, the Native Americans were able to get firearms from French and English settlers. Use of both horses and firearms significantly impacted Plains Indians. In particular, horses and firearms made hunting bison much easier, one of their main livelihoods. As a result, Plains Indians began spending less time as nomads dedicated to trapping beaver and instead turned to hunting bison.
What's more, the use of horses and firearms amounted to Plains Indians gaining more power. Many tribes began to stake territorial claims and become the dominant power in certain areas of the plains. For example, by the 1700s, the Osages became the dominant power in the areas of the lower Missouri and lower Red Rivers; by the late 1700s, the Comanches and their allies the Kiowas became the dominant power over Arkansas and the Red Rivers; and by the mid-1800s, the Sioux together with their allies the Arapahos and Cheyennes became the dominant power over "Minnesota and the Yellowstone Rivers in the north and the Republican River in the south" ("Native Americans").