The Native American tribe at the center of the controversy over the Dakota Access Pipeline (DAPL) has vowed to take legal action against a presidential memorandum by President Trump pushing pipeline construction.

After several months of protests led by the Standing Rock Sioux, the Army announced in early December that it would not approve an easement to let construction of the 1,172-mile Dakota Access Pipeline proceed along its planned route.

In a statement then, Assistant Army Secretary for Civil Works Jo-Ellen Darcy said alternate routes needed to be explored. Members of the tribe argue that the pipeline, which would carry nearly half a million barrels a day from the Bakken and Three Forks oil fields to a crude terminal near near Pakota, Ill., would damage sacred Native American sites and pollute their water.

The tribe’s reservation boundary is half a mile south of the proposed crossing under Lake Oahe, an Army Corps of Engineers project on the Missouri River in North Dakota.

“Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it’s clear that there’s more work to do,” Darcy said. “The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing.”

Trump’s Tuesday presidential memorandum directs the Army “to take all actions necessary and appropriate to review and approve in an expedited manner, to the extent permitted by law and as warranted, and with such conditions as are necessary or appropriate, requests for approvals to construct and operate the DAPL, including easements or rights-of-way to cross Federal areas” and “consider, to the extent permitted by law and as warranted, whether to rescind or modify the memorandum” issued by Darcy.

The order further directs that the Army “consider, to the extent permitted by law and as warranted, prior reviews and determinations, including the Environmental Assessment issued in July of 2016 for the DAPL, as satisfying all applicable requirements of the National Environmental Policy Act” and “review and grant, to the extent permitted by law and as warranted, requests for waivers of notice periods arising from or related to [U.S. Army Corps of Engineers] real estate policies and regulations” as well as “issue, to the extent permitted by law and as warranted, any approved easements or rights-of-way immediately after notice is provided to the Congress.”

Trump said in the Oval Office, while signing this presidential memorandum and another pushing construction of the Keystone XL pipeline, that “the process is so long and cumbersome that [companies] give up before the end.”

“Sometimes it takes many, many years and we don’t want that to happen,” Trump said. “And if it’s a no, we’ll give them a quick no. And if it’s a yes, it’s like let’s start building.”

Standing Rock Sioux David Archambault II said in a statement that the tribe is “not opposed to energy independence,” but to “reckless and politically motivated projects, like DAPL, that ignore our treaty rights and risk our water.”

“Creating a second Flint does not make America great again,” Archambault added, referencing the contaminated water crisis in Michigan.

The tribe said in a statement on their Facebook page that Trump’s order “not only violates the law, but it violates tribal treaties.”

“Nothing will deter us from our fight for clean water,” the Sioux vowed. “We will be taking legal action, and will take this fight head on.”

They encouraged supporters to submit comments for the Environmental Impact Statement in progress to help “compound our claim that the pipeline poses grave environmental risks.”

“Please also call your congressional representatives and let them know that the people do not stand behind today’s decision,” the tribe added. “Stand together as one and we will not fall.”

The day after Trump took office, the tribe asked DAPL protesters to abandon the site so that the land could be cleaned up before spring flooding.

“Moving forward, our ultimate objective is best served by our elected officials, navigating strategically through the administrative and legal processes,” said the tribal council in a statement. “…We ask the protectors to vacate the camps and head home with our most heartfelt thanks. Much work will be required to clean up before the spring thaw, which will flood the area. It is imperative we clean the camps and restore them to their original state before this flooding occurs.”