In 1851, the United States government signed a pair of treaties with the Dakota Sioux who ceded much of their land in the Minnesota Territory in return for goods and money. The Dakota neither received the full compensation nor all of the annuity payments: that which wasn't stolen by corrupt officials of the Bureau of Indian Affairs, was often paid straight to those traders with whom the Dakota had run up debts. Incursions onto their reservation and destruction of the ecosystem they relied on to survive left the Dakota impoverished and angry - anger that spilled over when the Federal government, distracted by the Civil War, was very late making the 1862 payment.
When the funds finally arrived in Minnesota, it was already too late. On 18th August, 1862, Little Crow, chief of the Mdewakanton Dakota Sioux, led a large party of braves in an attack on the Lower Sioux Agency, a settlement populated by Andrew Myrick, the Indian agent, and various other government officials. The braves killed ten people including Myrick, into whose mouth they stuffed grass as revenge for his response to their earlier request for assistance: 'Let them eat grass.'
The day before the attack, four Dakota braves killed five European settlers and stole food from them. The likelihood of violent reprisal against the Dakota persuaded Little Crow of the necessity of waging war on the settlers to drive them from the Minnesota River valley. The Dakota made further attacks until they suffered an overwhelming defeat at the Battle of Wood Lake in September, 1862.
Following their surrender, many of the Sioux faced a military tribunal without explanation of what was happening. With some trials lasting less than five minutes, 303 Sioux received a death sentence, although President Lincoln later commuted the majority of these. The US government forced the remaining Dakota Sioux from the Minnesota Valley to reservations in South Dakota.
The University of Missouri-Kansas School of Law site hosts a set of pages on the Dakota Conflict Trials 1862, which includes transcripts and analysis.
The funeral of Pocahontas: 21st March 1617