Treaty of Fort Laramie (1851) in the context of Fort Laramie National Historic Site

The Treaty of Fort Laramie (also the Sioux Treaty of 1868) is an agreement between the United States and the Oglala, Miniconjou, and Brulé bands of Lakota people, Yanktonai Dakota, and Arapaho Nation. The first Fort Laramie treaty, signed in 1851, had been a resounding failure. The United States had almost immediately violated it, with American miners and settlers immigrating into Indian territory. The United States Army had further broken the 1851 treaty when it initiated an armed invasion that resulted in the Grattan massacre in 1854, which sparked the first Sioux War. The 1868 treaty negotiations were thus initiated to replace the 1851 treaty, which had proven entirely ineffective.

The treaty is divided into 17 articles. It established the Great Sioux Reservation including ownership of the Black Hills, and set aside additional lands as "unceded Indian territory" in the areas of South Dakota, Wyoming, Nebraska, and possibly Montana. It established that the US government would hold authority to punish not only white settlers who committed crimes against the tribes but also tribe members who committed crimes and were to be delivered to the government, rather than to face charges in tribal courts. It stipulated that the government would abandon forts along the Bozeman Trail and included a number of provisions designed to encourage a transition to farming and to move the tribes "closer to the white man's way of life." The treaty protected specified rights of third parties not partaking in the negotiations and effectively ended Red Cloud's War. That provision did not include the Ponca, who were not a party to the treaty and so had no opportunity to object when the American treaty negotiators "inadvertently" broke a separate treaty with the Ponca by unlawfully selling the entirety of the Ponca Reservation to the Lakota, pursuant to Article II of this treaty. The United States never intervened to return the Ponca land. Instead, the Lakota claimed the Ponca land as their own and set about attacking and demanding tribute from the Ponca until 1876, when US President Ulysses S. Grant chose to resolve the situation by unilaterally ordering the Ponca removed to the Indian Territory. The removal, known as the Ponca Trail of Tears, was carried out by force the following year and resulted in over 200 deaths.

The Battle of the Little Bighorn, known to the Lakota and other Plains Indians as the Battle of the Greasy Grass, and commonly referred to as Custer's Last Stand, was an armed engagement between combined forces of the Lakota Sioux, Northern Cheyenne, and Arapaho tribes and the 7th Cavalry Regiment of the United States Army. It took place on June 25–26, 1876, along the Little Bighorn River in the Crow Indian Reservation in southeastern Montana Territory. The battle, which resulted in the defeat of U.S. forces, was the most significant action of the Great Sioux War of 1876.

Most battles in the Great Sioux War, including the Battle of the Little Bighorn, were on lands those natives had taken from other tribes since 1851. The Lakotas were there without consent from the local Crow tribe, which had a treaty claim on the area. Already in 1873, Crow chief Blackfoot had called for U.S. military actions against the native intruders. The steady Lakota incursions into treaty areas belonging to the smaller tribes were a direct result of their displacement by the United States in and around Fort Laramie, as well as in reaction to white encroachment into the Black Hills, which the Lakota consider sacred. This pre-existing Indian conflict provided a useful wedge for colonization and ensured the United States a firm Indian alliance with the Arikaras and the Crows during the Lakota Wars.

The Mandan, Hidatsa, and Arikara Nation (MHA Nation), also known as the Three Affiliated Tribes (Mandan: Miiti Naamni; Hidatsa: Awadi Aguraawi; Arikara: ačitaanu' táWIt), is a federally recognized Native American Nation resulting from the alliance of the Mandan, Hidatsa, and Arikara peoples, whose Indigenous lands ranged across the Missouri River basin extending from present day North Dakota through western Montana and Wyoming.

After the signing of the Fort Laramie Treaty (1851) and subsequent seizure of land, the Nation's land base is currently approximately 1 million acres located in Fort Berthold Reservation in northwestern North Dakota. The Tribe reported a total enrollment of 17,492 enrolled members of the Mandan, Hidatsa and Arikara Nation as of December 2024. Nearly 5,600 live on the Fort Berthold Indian Reservation; others live and work elsewhere.