Red Cloud's War (also referred to as the Bozeman War or the Powder River War) was an armed conflict between an alliance of the Lakota, Northern Cheyenne, and Northern Arapaho peoples against the United States and the Crow Nation that took place in the Wyoming and Montana territories from 1866 to 1868. The war was fought over control of the western Powder River Country in present-day north-central Wyoming and Montana.

In 1863, European Americans had blazed the Bozeman Trail through the heart of the traditional territory of the Cheyenne, Arapaho, and Lakota. It was the shortest and easiest route from Fort Laramie and the Oregon Trail to the Montana gold fields. From 1864 to 1866, the trail was traversed by about 3,500 miners, emigrant settlers and others, who competed with the Indians for the diminishing resources near the trail.

The United States named the war after Red Cloud, a prominent Oglala Lakota chief allied with the Cheyenne and Arapaho. The United States army had built forts in response to attacks on civilian travelers, using a treaty right according to historian Charles J. Kappler to "establish roads, military and other post". All three forts were located in 1851 Crow Indian territory and accepted by these Indians. The Crow believed they guarded their interests best by cooperating with the US army.

Red Cloud's War consisted mostly of constant small-scale Indian raids and attacks on the soldiers and civilians at the three forts in the Powder River country, wearing down those garrisons. The largest action of the war, the Fetterman Fight (with 81 men killed on the U.S. side), was the worst military defeat suffered by the U.S. on the Great Plains until the Battle of the Little Bighorn in the Crow Indian reservation ten years later. " ... the most dramatic battles between the army and the Dakota [in the 1860s and 1870s] were on lands those Indians had taken from other tribes since 1851."

With peace achieved under the Treaty of Fort Laramie in 1868, the Lakota and their allies were victorious. They gained legal control of the western Powder River country, took down the forts and permanently closed the Bozeman trail. The Crow lost their hunting grounds in the Powder River region to their enemies. With the treaty, "... the [United States] government had in effect betrayed the Crows, who had willingly helped the army to hold the posts for two years". The victory of the Lakota and their allies endured for eight years until the Great Sioux War of 1876, when the US resumed taking their territories, including the sacred Black Hills. In a 1980 Supreme Court case, United States v. Sioux Nation of Indians, the court ruled that tribal lands covered under the treaty had been taken illegally by the US government, and the tribe was owed compensation plus interest. As of 2018 this amounted to more than $1 billion.

As early as 1805, a Crow camp allowed French-Canadian fur trader François Antoine Larocque to follow it across parts of the Powder River area. According to him, the Crow "... inhabit the Eastern part of the Rocky Mountains at the head of the Rivière aux Roches Jaunes (Yellowstone River) and its Branches, Bighorn River, Tongue River, Powder River and others and Close to the head of the Missouri".

From the late 17th century, the Lakota had been moving west into the Plains, enlarging their territory so that by the early 19th century, they controlled the mid-Missouri River region. Cheyenne and Lakota warriors attacked a big Crow camp at Tongue River in 1820, known today as the Tongue River Massacre, making their enmity permanent. In 1843, United States explorer John Frémont said that Lakota had told him that they would soon fight the Crow and take their land, as the Crow country had about the best bison range in the west.

In 1851, the area in question was included in a treaty with the United States for the first time, namely in the Fort Laramie treaty. This peace agreement defined territories for the tribes, in an effort by the US to establish peace among Native tribes and protect its own settlers. The treaty was signed by representatives of some factions of the numerous tribes of the Plains and mountainous West, including Crow, Lakota, Cheyenne and Arapaho; Gros Ventre, Mandan, and Arikara; Assiniboine and other nations. Intertribal warfare had been common among the tribes to gain social and economic advantages in furs, slaves, horses, hunting grounds, and other interests. They continued to wage war against each other into the late 19th century, conducting affairs separate from interaction with US forces and representatives.