Various indigenous peoples lived in the territory of the present-day state of Montana for thousands of years. Historic tribes encountered by Europeans and settlers from the United States included the Crow in the south-central area, the Cheyenne in the very southeast, the Blackfeet, Assiniboine, and Gros Ventres in the central and north-central area, and the Kootenai and Salish in the west. The smaller Pend d’Oreille and Kalispel tribes lived near Flathead Lake and the western mountains, respectively. A part of southeastern Montana was used as a corridor between the Crows and the related Hidatsas in North Dakota.

The land in Montana east of the continental divide was part of the Louisiana Purchase in 1803. Subsequent to and particularly in the decades following the Lewis and Clark Expedition, American, British, and French traders operated a fur trade, typically working with indigenous peoples, in both eastern and western portions of what would become Montana. These dealings were not always peaceful, and though the fur trade brought some material gain for indigenous tribal groups, it also brought exposure to European diseases and altered their economic and cultural traditions.^[120] The trading post Fort Raymond (1807–1811) was constructed in Crow Indian country in 1807.^[121] Until the Oregon Treaty (1846), land west of the continental divide was disputed between the British and U.S. and was known as the Oregon Country. The first permanent settlement by Euro-Americans in what today is Montana was St. Mary’s (1841) near present-day Stevensville.^[122] In 1847, Fort Benton was established as the uppermost fur-trading post on the Missouri River.^[123] In the 1850s, settlers began moving into the Beaverhead and Big Hole valleys from the Oregon Trail and into the Clark’s Fork valley.