Police in riot gear forming a wall across a road. Water cannons spraying water protectors in the night. The movement to stop the Dakota Access Pipeline has yielded many arresting images. But what you don’t see is just as important.

It’s the legal battle underway as the Standing Rock Sioux Tribe tries to stop a pipeline that would cross ceremonial and burial grounds, and also put a sole source of drinking water at risk. A federal court ruled earlier this year that the Army Corps of Engineers “has likely complied” with federal obligations to preserve these sites sacred to the Standing Rock Sioux. But the Corps still halted construction to review its handling of the process. And then the company building the pipeline sued to restart construction.

These lawsuits hinge on questions around tribal sovereignty, the body of law that steers tribal nations’ relationship with the United States government.