The Assiniboine and Sioux had both gradually been pushed westward onto the plains from the woodlands of Minnesota by the Ojibwe, who had acquired firearms from their French allies.

Later, the Assiniboine had acquired horses via raiding and trading with neighboring tribes of Plains tribes such as the Crow and the Sioux on their south.

The Assiniboine eventually develop into a large and powerful people with a horse and warrior culture; they use the horse to hunt the vast numbers of bison that live within and outside their territory.

At the height of their power, the Assiniboine dominate territory ranging from the North Saskatchewan River in the north to the Missouri River in the south, and including portions of modern-day Saskatchewan, Alberta, and Manitoba, Canada; and North Dakota and Montana, United States of America.
 
The first person of European descent to describe the Assiniboine is an employee of the Hudson's Bay Company named Henry Kelsey in the 1690s.

Later explorers and traders Jean Baptiste de La Vérendrye and his sons (1730s), Anthony Henday (1754–55), and Alexander Henry the younger (1800s) confirm that the Assiniboine hold a vast territory across the northern plains, including into the United States (which achieves independence in 1776 but does not acquire the plains until 1803 in the Louisiana Purchase from France.)

Pierre Gaultier de Varennes, sieur de La Vérendrye, the son of Rene Gaultier, Sieur de Varennes, the longtime governor of Trois Rivieres, had become in 1726 the partner of his brother Jacques Rene in the fur trade north of Lake Superior, where he has net Amerinds whose tales persuade him that a water route through the lakes and rivers of the northwest leads to the storied Western Sea.

Vérendrye, an ex-soldier, had farmed and fur traded to support his family until 1727 when he moved to Nipigon to command that fort.

He had in 1728 been appointed commandant of the French posts on the north shore of Lake Superior, and in in 1731 begins his explorations in earnest.

One of the main objectives is to find a route to the Western Sea.

The governor and intendant of New France backs Vérendrye, accompanied by three of his sons and a nephew, in establishing a post at Rainy Lake, the first of several such posts along an intended route toward the western ocean.

The first record of encounter with the Mandan by Europeans is in 1738 with the visit of the French Canadian trader La Vérendrye in 1738.

The Mandans carry him into their village, whose location is unknown.

It is estimated that at the time of his visit, fifteen thousand Mandan reside in the nine well-fortified villages on the Heart River; some villages have as many as a thousand lodges.

According to Vérendrye, the Mandans at this time are a large, powerful, prosperous nation who are able to dictate trade on their own terms.

They trade with other Native Americans both from the north and the south, from downriver.

Their friendliness and willingness to trade will bring many traders and fur trappers to their villages over on the Upper Missouri in the next century.

Speakers of Mandan, a Siouan language, the people have develop a settled culture in contrast to that of more nomadic tribes in the Great Plains region.

They have established permanent villages featuring large, round, earthen lodges some forty feet in diameter, surrounding a central plaza.

While the bison is key to the daily life of the Mandan, it is supplemented by agriculture and trade.
Archaeological research suggests the Mandan people migrated from the Ohio River valley to the banks of the upper Missouri River.

La Vérendrye has established posts at Rainy Lake, the Lake of the Woods, Lake Winnipeg, the Red River, and the Assiniboine River by 1738.

Although unable to discover a river flowing toward the fabled Western Sea, his impressive gains as a fur-trader persuade the French minister of the marine, Jean Frederic, comte de Maurepas, that his true interest is commercial gain, over the advancement of knowledge.

La Vérendrye visits the Mandan villages on the Missouri River in modern North Dakota late in the year, entering the main village accompanied by six hundred Assiniboines and thirty Mandans flying the French colors and beating drums.

An Arikara village, near where present-day Pierre, South Dakota will develop, is visited in 1743 by two sons of the French trader and explorer La Vérendrye.

The Arikara's name is believed to mean "horns," in reference to the ancient custom of wearing two upright bones in their hair.

The name also could mean "elk people" or "corn eaters."

The Arikara language is a member of the Caddoan language family. Arikara is close to the Pawnee language, but they are not mutually intelligible.

Linguistic divergence between Arikara and Pawnee suggests a separation from the Skidi Pawnee in about the fifteenth century.

The Arzberger Site near present-day Pierre, South Dakota, designated as a National Historic Landmark, is an archeological site from this period, containing the remains of a fortified village with more than forty-fou rlodges.

La Verendrye, a financial failure despite the establishment of numerous trading posts on the buffalo plains, had resigned in 1744, leaving his sons and nephews to carry on the western commerce.

During his years of activity, La Veredrye has won new Amerind nations to French allegiance and lured their trade away from the Hudson’s Bay Company and toward Montreal.

The new French minister of the marine, Antoine Louis Rouille, has him decorated with the cross of the order of Saint Louis in 1745; La Verendrye dies in December of the same year.