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 s**t 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 f*****g 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).