One of the Mormons had a crippled cow, which at some point got spooked and decided to wander off into the wilderness. Its owner started to follow but changed his mind when he noticed a ton of goddamn Sioux warriors in the surrounding area. Roughly four-thousand members of the tribe had gathered around the nearby Ft. Laramie to trade. The cow spent several hours doing cow-things before a few of the younger Sioux noticed that dinner had just stumbled into camp and killed the shit out of it.
That very next day, the Mormons arrived at Ft. Laramie and complained to the soldiers about the poached cow. Twenty-nine men and two cannons, lead by West Point graduate Lieutenant Grattan, marched out to talk with the Brule Sioux and seek justice for the cow--because nothing says "measured discussion" like a regiment with fucking cannons. None of the men spoke Sioux, but they had the foresight to bring along a French trader named Lucien Auguste, who spoke a completely different Indian dialect. Poorly.
They met with the Sioux Chief, Conquering Bear, and ordered him to turn over the cow's murderer, the spectacularly-named High-Forehead. Grattan was young, brash and eager for a fight, while Lucien Auguste was hammered and purposefully mistranslated much of what both men said (presumably because it seemed hilarious at the time).