The Iron Confederacy or Iron Confederation (also known as Cree-Assiniboine in English or Nehiyaw-Pwat in Cree) was a political and military alliance of Plains Indians of what is now Western Canada and the northern United States. This confederacy included various individual bands that formed political, hunting and military alliances in defense against common enemies. The ethnic groups that made up the Confederacy were the branches of the Cree that moved onto the Great Plains around 1740 (the southern half of this movement eventually became the "Plains Cree" and the northern half the "Woods Cree"), the Saulteaux (Plains Ojibwa), the Nakoda or Stoney people also called Pwat or Assiniboine, and the Métis and Haudenosaunee (who had come west with the fur trade). The Confederacy rose to predominance on the northern Plains during the height of the North American fur trade when they operated as middlemen controlling the flow of European goods, particularly guns and ammunition, to other Indigenous nations (the "Indian Trade"), and the flow of furs to the Hudson's Bay Company (HBC) and North West Company (NWC) trading posts. Its peoples would take part in the bison (buffalo) hunt, and the pemmican trade. The decline of the fur trade and the collapse of the bison herds sapped the power of the Confederacy after the 1860’s.

The Assiniboine are believed to have originated on the southern edge of the Laurentian Shield in present-day Minnesota. They became a separate people from their closest linguistic cousins, the Yanktonai Dakota, sometime prior to 1640 when they are first mentioned by Europeans in the Jesuit Relation. They were not a member of the "Seven Fires Council" of the Great Sioux Nation by this time and were referred to by other Sioux speakers as the Hohe or "rebels". By 1806, the historical evidence definitively locates them in the Assiniboine River valley in present-day Saskatchewan and Manitoba.

The Cree had been in contact with Europeans since around 1611 when Henry Hudson reached their ancestral homeland around Hudson and James Bays. The traditional view of historians, based on the accounts of European traders, is that once the Hudson's Bay Company (HBC) began to establish itself in the Hudson Bay region, two branches of the Cree began moving west and south to act as middlemen traders. They denied other plains peoples access to the HBC, except for the Assiniboine, in exchange for peaceful relations. A more recent view, based on oral history and linguistic evidence, suggests that the Cree were already established west of Lake Winnipeg when the HBC arrived, and were likely present as far west as the Peace River Region of present-day Alberta.

When the Hudson's Bay Company opened its first bayside posts in 1668 and 1688, the Cree became their main customers and resellers. Prior to this the Cree had been at the northwestern edge of a trade system linked to the French, from which they received only the secondhand goods others were ready to discard. Once in possession of direct access to European tools and weapons, the Cree were able to expand rapidly West.

The earliest written record of the military and political relations of the nations west of Hudson's Bay comes from Henry Kelsey's journal c. 1690–1692. In it, he states that the Cree and the Assiniboine had good relations with the Blackfoot and were already allies against the "Eagle Birch Indians, Mountain Poets, and Nayanwattame Poets" (the identities of these groups are uncertain but they may have been other Siouan-speakers, or Gros Ventres).

The history of the Stoney before the mid-eighteenth century are obscure. They speak a Siouan language they call nakoda, which is little different from Assiniboine. The present-day Stoney Nation of Alberta believes that Kelsey's mention of the "Mountain Poets" may refer to their ancestors. However, the consensus view is that they were not yet a separate people from the Assiniboine. There is clear evidence of them as a separate group from 1754–1755 when Anthony Henday wrote of camping with "Stone" families near present-day Red Deer, Alberta. The Stoney were already trading with the Cree fur traders at this point and were military allies.

American ethnographer and historian Edward S. Curtis wrote about the close but unstable relationship between the Assiniboine and the Plains Cree, and how, after the Plains and Woods Cree territories diverged, the Woods Cree were no longer a part of this military alliance:

The neighbors of the western Cree were Athapascans on the north and northwest, Blackfeet on the west, and Assiniboine on the south. With the Assiniboine they were closely associated from the time of the separation of that tribe from the parent Sioux prior to the opening of the country by exploration in the early years of the seventeenth century; nevertheless, there were rather frequent drunken brawls, with consequent murders, between the two tribes in the boisterous era of the fur-trade. They joined forces in pushing the Blackfeet, Bloods, and Piegan southwestward out of the plains bordering Saskatchewan river, and up to the termination of inter-tribal warfare remained constant enemies of these other Algonquians. The Cree inheritance of the historic Sioux hostility toward the Chippewa was not lessened by the friendly reception they accorded the renegade Assiniboine, for whom the Sioux entertained bitter hatred mixed with professed contempt. The Woods Cree had little, if any, part in this warfare with the Blackfeet and the Sioux; their operations were limited to dispossessing the Athapascans of their territory between the Saskatchewan and Athabasca lake. Peace river, according to Henry, received its name from the circumstance that the Cree and the Beavers settled their hostilities at Peace point.