The Army Corps of Engineers approved a key easement Wednesday night to allow the completion of the 1,100-mile Dakota Access oil pipeline, as native groups protested outside the White House to kick off the next phase of mass resistance against the project.

The Standing Rock Sioux Tribe led an intense protest campaign last year to prevent the company building the pipeline, Energy Transfer Partners, from building a short segment of the project under Lake Oahe. The tribe argued that the oil pipeline would jeopardize their only source of freshwater.

The Army Corps approved the mile-long easement required to go under the lake this evening, effectively reversing the previous administration's actions to halt the $3.8 billion project in order to do a new environmental assessment.

"The safety of those located on Corps-managed land remains our top priority, in addition to preventing contaminants from entering the waterway," said Col. John Henderson for the Army Corps. The Army had been coordinating with the tribes to prepare the site for construction to begin, he said.

"We appreciate the proactive efforts of the tribes to help clean the protest site ahead of potential flooding along the river, typical during the runoff season," Henderson said.

But tribal groups also amassed on the grounds near the White House Wednesday night to tell President Trump that the next phase of their fight against the pipeline has begun.

"Today begins the next phase of mass resistance to Donald Trump's toxic Dakota Access Pipeline," said Dallas Goldtooth, director of the Indigenous Environmental Network, in a statement given at the protest.

"This is our land, our water, our health, and our culture at stake — and if Donald Trump thinks we will give all of that up without a fight he is wrong."

Wednesday night's decision came two weeks after Trump issued an executive order for the Army Corps to expedite approval of the pipeline.