system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The 1851 treaty that keeps being mentioned in the North Dakota pipeline protest.

Explain  like I'm a non-american five year old too.
gpt
Basically, The government fought the Standing Rock Sioux tribe among others for control of the territory that is now North and South Dakota. The government then signed a treaty saying "this bunch of the land is the tribes" while the rest of it was controlled by the US. 

The tribe argues that the pipeline (which falls on government land) endangers drinking water sources on land that belongs to the tribe.