The Arikara War was a military conflict between the United States and Arikara in 1823 fought in the Great Plains along the Upper Missouri River in the Unorganized Territory (presently within South Dakota). For the United States, the war was the first in which the United States Army was deployed for operations west of the Missouri River on the Great Plains. The war, the first and only conflict between the Arikara and the U.S., came as a response to an Arikara attack on U.S. citizens engaged in the fur trade. The Arikara War was called "the worst disaster in the history of the Western fur trade".

Relations between the United States and the Arikara commenced in 1804. Initially, the relations were amicable. In 1806, Ankedoucharo, an Arikara leader, died during a trip to the United States capital. Although the U.S. claimed that his death was from natural causes, the Arikara widely believed that Ankedoucharo was deliberately murdered by U.S. citizens. In subsequent years, contact between the Arikara and White Americans increased as a result of the growing activity of corporations engaged in the international fur trade. In early 1823, the Arikara attacked a corporate fort belonging to the Missouri Fur Company, killing two U.S. citizens.

The Arikara had poor relations with the Western Dakota and Lakota Sioux, two stronger Indigenous nations along its borders. The Arikara involved in the war were living in two communities on the west shore of the Missouri River located approximately six miles upstream from the mouth of Grand River, and a small creek separated the two fortified communities of Arikara homes.

The causes of the war are not well recorded, but the trading relationship of the Arikara with white traders was certainly a factor. The Arikara lived in permanent settlements for most of the year where they farmed, fished and hunted buffalo on the surrounding plains. However, this was insufficient to sustain them and they relied on being a center of trade with neighboring tribes to survive. William Henry Ashley's expedition to directly acquire furs and pelts cut out the Arikara in their role as trading middle-men and was thus a direct threat to their livelihood. There was also the issue of their desire to have a trading post on their territory so that they could have easy access to manufactured goods. They resented the fact that their long-time enemies, the Sioux, had such posts, but they did not. Ashley had been asked to set up a trading post when he was in the area in 1822. Not wishing to limit his operations by having to maintain a permanent base, Ashley instead promised the Arikara that he would have the goods they asked for shipped to them directly from St. Louis. Ashley had not made good this promise at the time of his 1823 expedition, and possibly never intended to. The death of Ankedoucharo was probably not a direct cause of the war, but it did add to the general resentment.

On 2 June 1823, Arikara warriors assaulted trappers working for Ashley's Rocky Mountain Fur Company on the Missouri River, killing about 15 people. The surviving trappers retreated down the river and hid in shelters, where they stayed for more than a month.^{[citation needed]}

The United States responded with a combined force of 230 soldiers of the 6th Infantry, 750 Sioux allies, and 50 trappers and other company employees under the command of Lieutenant Colonel Henry Leavenworth, Fort Atkinson, present-day Nebraska:

"The forces thus organized, including regular troops, mountaineers, voyageurs and Indians, were styled the Missouri Legion."

The 750 warriors were part Yankton and Yanktonai Sioux, part western Sioux from the Brule, the Blackfeet, and the Hunkpapa divisions. The Lakotas "... appeared anxious to join us". The Indian force received promises of Arikara horses and spoils, and with the enemy's villages fallen new ranges would open for the Sioux.^{[citation needed]}