The Sioux Wars is an American event, that marks the beginning of the event chain known as the the Sioux Wars. The event leads the way to the events Little Bighorn and End of the Sioux Wars eventually concluding in Dakota being an accepted culture.
It can happen after 
USA has taken the decision Expedition to The Black Hills, which they can take after 1874 if they own Rapid City, South Dakota and have a unit stationed there. The decision gives 1 prestige.
The event The Sioux Wars can happen if USA still owns Rapid City and the RGO has changed to 
precious metal. It has a mean time to happen of 1 year.
It gives all Dakota POPs a militancy of 8. 8 militancy is quite a lot, but on the other hand Dakota is a tiny part of the American population and USA should easily be able to deal with eventual rebels.
The Sioux wars can end with the event End of the Sioux Wars, if there are no rebels in the country. It has a mean time to happen of 24 months, and decreases militancy by 10 and adds Dakota as an accepted POP. It also kills 50% of the remaining Dakota POP.
Ingame description
With the influx of settlers into the Black Hills area, the conflicts with the local Sioux has excalated. Today, news reached us of a massive War Party, led by an indean whose name reports only give as 'Sitting Bull', has risen up in the Dakota territory.