Before the Great War, Spearfish was a relatively small town in South Dakota. Despite being unknown in comparison to bigger cities (and the nearby area of Spearfish Canyon, known for its nature), Spearfish was generally considered "quaint" and underrated by many tourists. The town was not hit by the New Plague.
When the bombs fell, the town quickly suffered from a lack of clean water. Many inhabitants, desperate, started drinking the irradiated water flowing from Spearfish Canyon. It can be speculated that the water may have been uniquely irradiated, since the water caused many people to lose their better judgement, and seldom their higher cognitive abilities.
By 2104, some inhabitants also started living in Spearfish Canyon, and they eventually became the "Spearfish" tribe. Meanwhile, the town itself suffered from inbreeding, making their problems with intelligence and socializing even worse. Despite some of the inhabitants' optimism and enthusiasm, much of the town became reclusive, and some areas became battlegrounds between territorial families.
In 2115, the Spearfish tribe came into contact with the Dakota. More specifically, a small Dakota scout group. Since many of the Spearfish were itching to trade with someone outside their group, they took a risk and tried to converse with the Dakota, which didn't end well. The Spearfish, despite appearing to be another promitive tribe with unremarkable weapons, actually had a supply of hunting rifles, albeit in poor condition. This allowed the Spearfish to claim victory over the small Dakota band, although eight of their men were killed and sixteen were wounded in the process. Since this encounter, the Dakota have been sworn enemies to the Spearfish.
Many, many years later, in 2281, a small group of American Northern Army soldiers approached the Spearfish chieftain's hut. They proposed an alliance, since the Dakota, who are aligned with Custer, may still be vengeful after the scouts' death. Plus, Spearfish Canyon could be a great place to settle for the soldiers. The leader refused, stating that he wouldn't allow them to turn his peaceful land into "an ugly marsh of meaningless war". The soldiers left camly, although somewhat irritated.