The Dakota War of 1862, also known as the U.S.-Dakota War of 1862, the Sioux Uprising, the Dakota Uprising, the Sioux Outbreak of 1862, the Dakota Conflict, or Little Crow's War, was an armed conflict between the United States and several eastern bands of Dakota collectively known as the Santee Sioux. It began on August 18, 1862, when the Dakota, who were facing starvation and displacement, attacked the Lower Sioux Agency and white settlements along the Minnesota River valley in southwest Minnesota. The war lasted for five weeks and resulted in the deaths of hundreds of settlers and the displacement of thousands more. In the aftermath, the Dakota people were exiled from their homelands, forcibly sent to reservations in the Dakotas and Nebraska, and the State of Minnesota confiscated and sold all their remaining land in the state. Thirty-eight Dakota men were subsequently hanged for crimes committed during the conflict in the largest mass execution in US history.

All four bands of eastern Dakota had been pressured into ceding large tracts of land to the United States in a series of treaties and were reluctantly moved to a reservation strip twenty miles wide, centered on the Minnesota River. There, they were encouraged by U.S. Indian agents to become farmers rather than continue their hunting traditions. A crop failure in 1861, followed by a harsh winter along with poor hunting due to depletion of wild game, led to starvation and severe hardship for the eastern Dakota. In the summer of 1862, tensions between the eastern Dakota, the traders, and the Indian agents reached a breaking point. On August 17, 1862, four young Dakota men killed five white settlers in Acton, Minnesota, after a disagreement. That night, a faction led by Chief Little Crow decided to attack the Lower Sioux Agency the next morning in an effort to drive all settlers out of the Minnesota River valley. The demands of the Civil War slowed the U.S. government response, but on September 23, 1862, an army of volunteer infantry, artillery and citizen militia assembled by Governor Alexander Ramsey and led by Colonel Henry Hastings Sibley finally defeated Little Crow at the Battle of Wood Lake. Little Crow and a group of 150 to 250 followers fled to the northern plains of Dakota Territory and Canada.

During the war, Dakota men attacked and killed over 500 white settlers, causing thousands to flee the area and took hundreds of "mixed-blood" and white hostages, almost all women and children. By the end of the war, 358 settlers had been killed, in addition to 77 soldiers and 36 volunteer militia and armed civilians. The total number of Dakota casualties is unknown, but 150 Dakota men died in battle. On September 26, 1862, 269 "mixed-blood" and white hostages were released to Sibley's troops at Camp Release. Interned at Fort Snelling, approximately 2,000 Dakota surrendered or were taken into custody, including at least 1,658 non-combatants, as well as those who had opposed the war and helped to free the hostages.

In less than six weeks, a military commission, composed of officers from the Minnesota volunteer infantry, sentenced 303 Dakota men to death. President Abraham Lincoln reviewed the convictions and approved death sentences for 39 out of the 303. On December 26, 1862, 38 were hanged in Mankato, Minnesota, with one getting a reprieve, in the largest one-day mass execution in American history. The United States Congress abolished the eastern Dakota and Ho-Chunk (Winnebago) reservations in Minnesota, and in May 1863, the eastern Dakota and Ho-Chunk imprisoned at Fort Snelling were exiled from Minnesota to a reservation in present-day South Dakota. The Ho-Chunk were later moved to Nebraska near the Omaha people to form the Winnebago Reservation.

In 2012 and 2013, Governor Alexander Ramsey's 1862 call for the Dakota to "be exterminated or driven forever beyond the borders of the State" was repudiated, and in 2019, an apology was issued to the Dakota people for "150 years of trauma inflicted on Native people at the hands of state government."

The eastern Dakota were pressured into ceding large tracts of land to the United States in a series of treaties negotiated with the U.S. government and signed in 1837, 1851 and 1858, in exchange for cash annuities, debt payments, and other provisions. Under the terms of the Treaty of Traverse des Sioux signed on July 23, 1851, and Treaty of Mendota signed on August 5, 1851, the Dakota ceded large tracts of land in Minnesota Territory to the U.S. in exchange for promises of money and supplies.

The treaties of Traverse des Sioux and Mendota committed the Dakota to live on a 20-mile (32  km) wide reservation centered on a 150 mile (240 km) stretch of the upper Minnesota River. During the ratification process, however, the U.S. Senate removed Article 3 of each treaty, which had defined the reservations. In addition, much of the promised compensation went to traders for debts allegedly incurred by the Dakota, at a time when unscrupulous traders made enormous profits on their trade. Supporters of the original bill said these debts had been exaggerated.

When Minnesota became a state in 1858, representatives of several Dakota bands led by Little Crow traveled to Washington to negotiate about implementing the existing treaties. Instead, the Sioux lost the northern half of the reservation along the Minnesota River in the resulting 1858 Dakota Treaty. This loss was a major blow to the standing of Little Crow in the Dakota community.