#askU2 -Did you realize DAPL's installation is probably illegal? -this funds @RED. #askwarren
There are prominent legal issues on not one, but (at least) two counts, the foremost being the question of whether it's a Treaty violation. It is also illegal to revoke an EIS (Environmental Impact Statement) based on a Presidential Memorandum, especially when you deep six the report that says you had a legal basis for launching the EIS in doing so. There are new, more substantive articles on this, as well as a new report by the UN on the matter.
Violating a Treaty isn't just violating the law, it's a violation of Article VI of the U.S. Constitution.
US Federal Court has now ruled on this, by the way. They've just allowed oil to continue to flow despite finding the pipeline's implementation illegal on several counts.
"The Standing Rock Sioux Claim ‘Victory and Vindication’ in Court
A federal judge rules that the Dakota Access pipeline did not receive an adequate environmental vetting." - The Atlantic
Feds Urge Judge to Keep Oil Flowing in Dakota Access Pipeline During Environmental Review - Ecowatch
To help you identify the bounty of the information trove you've just landed on, the issues that this judge deemed illegal were brought up to substantiate Question #6 (namely the points numbered 6 and 7). Not only were they specifically addressed in the ruling (though the Judge has deemed the USACE's reversal on the EIS to have not been illegal, which would take it right up to the president) - it seems clear that the identified issue of specifically withholding spill modelling and other assessments from the tribe, and how this violated their Treaty rights, was indeed at issue.