Following a boost from the Trump administration, the U.S. Army Corps of Engineers (Corps) on Tuesday returned to its original assessment and granted an easement to the backers of the controversial $3.8 billion, 1,200-mile Dakota Access Pipeline (DAPL) for its final leg, a water crossing in south-central North Dakota.

Although opponents said they would file in court to try to block construction of the nearly complete DAPL project, construction could begin almost immediately on a crossing under a dammed portion of the Missouri River forming Lake Oahe near the Standing Rock Sioux Tribe’s reservation. The Sioux and a collaborative of environmental activists and other Native American tribes have been opposing the final crossing for months.

Native American officials plan to fight the action in the courts and with massive protests.

“Donald Trump will not build his Dakota Access Pipeline without a fight,” said Tom Goldtooth, executive director of the Indigenous Environmental Network. “The granting of an easement, without any environmental review or tribal consultation, is not the end of this fight — it is the new beginning. Expect mass resistance far beyond what Trump has seen so far.”

Although DAPL sponsors, led by Houston-based Energy Transfer Partners (ETP), had gained all regulatory approvals and won all legal challenges in federal court last year, the Obama administration reversed the Corps’ earlier action and late last year ordered further environmental review of the project, which had already gone through an extensive regulatory process by state and federal authorities going back to 2014.

Supporters of the project, the Midwest Alliance for Infrastructure Now (MAIN), said the Trump administration and the Corps “followed the letter of the law” in freeing up the four-state project for transporting the bulk of Bakken Shale crude oil production in North Dakota to domestic and foreign markets through a hub in central Illinois.

Tuesday’s “action sends a strong positive signal to those individuals and companies seeking to invest in the United States, and will help strengthen our economy and create jobs,” said MAIN spokesperson Craig Stevens.

Lawyers for the Standing Rock Sioux have been insisting that the Trump administration could not overturn the decision to begin a new environmental assessment on the project in the waning days of the Obama administration.

The easement now allows ETP to build the passage under the federally owned lake. On Monday, the company told a federal judge the work could be completed in about 60 days.