Opponents of the Dakota Access Pipeline won an important victory on Sunday when the U.S. Army Corps of Engineers denied the final permission to cross Lake Oahe near the Standing Rock Sioux Reservation. But even as Native American protesters celebrated, they acknowledged the larger battle is far from won.

The Army Corps’ decision keeps the pipeline from crossing a spot the tribe has vowed to protect, and called for a more comprehensive environmental review of alternative routes. But an incoming Trump administration could not only reverse this decision, but could also strike back at a core environmental law that has governed fights like this for more than half a century.

There are many in the oil industry, Congress and President-elect Donald Trump’s inner circle ready to target the process of repeated reviews under the National Environmental Policy Act (NEPA). That law resulted in the years-long review of the Keystone XL tar sands pipeline, eventually ending in President Obama’s rejection of a cross-border permit.

The question is how aggressively, and how soon, they will move to rein in the law, a potent tool for two generations of environmentalists.

The Dakota Access Pipeline’s opponents are happy to claim their victory, even if it is only temporary. For months, they have gathered under the banner of water protectors, facing down forceful tactics by authorities while demanding relief from Washington.

“While this is clearly a victory, the battle is not ‘over,’” said a statement from four organizations supporting the encampment: Honor the Earth, the International Indigenous Youth Council, the Indigenous Environmental Network and the Sacred Stone Camp.

“The Trump administration could easily approve the project early next year,” they warned. “The Obama Administration has never guaranteed the water protectors or the Standing Rock Sioux Tribe that they would use force to stop Dakota Access from drilling under the river without a permit, if necessary. The Army Corps has not yet agreed to pursue a full EIS for the entire length of the pipeline.”

The companies constructing the pipeline, Energy Transfer Partners and Sunoco Logistics Partners, called the delay “just the latest in a series of overt and transparent political actions by an administration which has abandoned the rule of law in favor of currying favor with a narrow and extreme political constituency.

They said they “are fully committed to ensuring that this vital project is brought to completion and fully expect to complete construction of the pipeline without any additional rerouting in and around Lake Oahe. Nothing this Administration has done today changes that in any way.”

The Corps announcement on Sunday was couched in conciliatory if bureaucratic language, but was one of the most provocative acts of the outgoing administration in the face of the Trump arrival.

“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,” Assistant Secretary of the Army Jo-Ellen Darcy said in a statement. “The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing.”

Activists tempered their celebratory words with a nod to what may lie ahead.

“Trump, of course, can try and figure out a way to approve the pipeline right away, though the Obama administration has done its best to make that difficult,” Bill McKibben, co-founder of the climate activist group 350.org, wrote in The Guardian.  “If Trump decides to do that, he’s up against people who have captured the imagination of the country.”

He said the Standing Rock Sioux and allied protesters “managed to build not just resistance to a project, but a remarkable new and unified force that will, I think, persist … even in the face of the new Trump administration.”

Leading Congressional Republicans denounced the move. House Speaker Paul Ryan called it “big government decision-making at its worst,” saying on Twitter that he looks forward to “putting this anti-energy presidency behind us.” Rep. Kevin Cramer, a Republican of North Dakota who is a  Trump energy adviser, called it a “chilling symbol.”

Another key player on the Trump transition team, Thomas Pyle of the Institute for Energy Research, a group partly funded by the Koch brothers network, called the decision “a violation of the rule of law that will chill investment in safe energy infrastructure.”

There are many ways Republicans in Congress could work with the new administration to push ahead the project and others like it.

They could pass a bill to deem the project in compliance with environmental statutes such as the Clean Water Act, the Endangered Species Act and NEPA. That might face a filibuster by minority Democrats in the Senate.

More broadly, it might jumpstart wholesale revisions to NEPA, signed into law by President Nixon. It has been a main tool for environmentalists fighting what they consider misguided federal development projects and a longtime target of pro-development conservatives.

William Yeatman, a senior fellow at the the Competitive Enterprise Institute, a conservative advocacy organization that is influential on the Trump team, said the decision “will focus attention on NEPA and particularly, how it can be abused as a means to an end.”

Republicans, in their 2016 platform, pledged to “modernize the permitting process under the National Environmental Policy Act so it can no longer invite frivolous lawsuits, thwart sorely needed projects, kill jobs, and strangle growth.”

Anna Aurilio, director of the Washington, D.C. office of Environment America, said this fight was coming regardless of Dakota Access.

“I don’t think the GOP needs any more or less fodder to attack NEPA,” she said. “They have this and other environmental regs in their cross hairs. They have an all-of-the-above environmental rollback strategy. They have had this on their hit list for a long time.”

In each of the last three Congresses, dozens of bills have been introduced by Republicans to undermine NEPA, according to Raul Garcia, legislative counsel for Earthjustice who works on the defense of NEPA. (Earthjustice represents the Standing Rock tribe in some of its litigation.)

Cutting back NEPA has long been a top priority of Rep. Rob Bishop (R-Utah), chairman of the House Natural Resources Committee. Senator John Barrasso (R-Wyo.), expected to lead the Environment and Public Works Committee, introduced legislation to exempt certain types of pipelines from the NEPA process.

Some believe the GOP will not launch a head-on attack to dismantle NEPA, but rather try to bring about death by a thousand cuts. For example, they want to overturn Obama directives requiring climate to be considered in NEPA reviews, and urging agencies to consider the costs of future climate damages alongside the benefits of building projects.

“There are a bunch of ways they can subvert or reverse Obama’s decision,” said Joel Mintz, law professor at Nova Southeastern University in Ft. Lauderdale and a scholar at the Center for Progressive Reform. “For example, NEPA doesn’t require a substantive result as much as a ‘hard look’ at environmental impacts. The Trump administration could say they looked at the environmental consequences but they consider the energy needs of the country to be greater, and so they proceed with the original pipeline route.”

Kevin Book, managing director of ClearView Energy Partners, a research and consulting firm that advises energy companies and investors, wrote to clients that even though the Corps did not rule out Dakota Access’ planned pipeline route, “we expect that the pathways to getting there may take longer.” He said expected the delay to last at least into the second half of 2017.

Had the Obama administration simply denied the easement, it would have given Dakota Access a “final agency action” it could appeal in court. Instead, the pipeline company doesn’t have that option.

If the Trump administration moves to reverse Obama administration’s decision, it could be forced to defend in court why there should be no study of alternative pipeline routes, or potential impacts on the tribe’s hunting and fishing rights, or the extent and location of the tribe’s treaty rights—three issues the Corps said that an Environmental Impact Statement was needed to address.

An environmental impact statement would take at least six to 12 months. Under NEPA, agencies are required to identify and invite the participation of interested parties to a decision, and the process typically involves public meetings, formal hearings or informal workshops.

ICN’s Neela Banerjee and Marianne Lavelle contributed reporting to this story.