The Spanish, in founding of the Northern New Mexico colony in 1598, had introduced the Puebloan peoples to new farming implements and provided some measure of security against Navajo and Apache raiding parties.

The Tiwa and Tewa speakers of the New Mexican Pueblos, tending the horses of their Spanish overlords, learn to care for, breed and, eventually, ride horses.

Though the Spaniards have decreed native ownership of horses illegal, the Apache, Kiowa and Utes begin raiding for horses in the early seventeenth century.

The Assiniboine, along with the Stoney of Alberta, share a common ancestry with the Sioux nation.

While it was formerly believed that the Assiniboine originated among the Yanktonai division of the Dakota Sioux, linguistic analysis indicates that the Assiniboine and Stoney together form a group coordinate with that of the Santee, Lakota, and Yankon-Yanktonai, and that they are no more related to one of these subdivisions than another.

The separation of the Assiniboine from the Sioux must have occurred at some time prior to 1640, as Paul Le Jeune names them along with the "Naduessi" (Sioux) in his Jesuit Relations of this year.

Trading between Spanish settlers and natives is rare and occurs in parts of New Mexico and California.

The Spanish mainly intend to spread the Christian faith to natives and to use them as slaves for work.

The most significant effect of trading with the Spanish is the introduction of the horse to the Ute in New Mexico.

Gradually, horses breed and their use is adopted across the Great Plains, dramatically altering the lifestyles and customs of many Native American tribes.

Many natives switch from a hunter-gatherer economy to a nomadic lifestyle after they begin using horses for transportation.

They have a greater range for hunting bison and trading with other tribes.

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.)

Kiowas, who live mostly in north Texas, Oklahoma and eastern New Mexico at the time of the arrival of Europeans, sell horses to ...

...Wichitas and, later, to ...

...Cheyennes and Arapahos.

Utes trade horses to ...

...the Shoshoni of Wyoming.

The Shoshonis trade with the Absarokes, who have separated from the Hidatsa, and to ...