Legally, the Dakota Access pipeline is closer to completion than it has been in months.

In his first three weeks in office, President Donald Trump bucked norms about executive propriety and canceled an environmental-impact review ordered by the Obama administration. On Wednesday, the U.S. Army Corps of Engineers granted the final easement for the pipeline’s developer, Energy Transfer Partners, to begin construction beneath Lake Oahe, near Cannon Ball, North Dakota.

The U.S. Army Corps’s move was designed to speed things up. Normally, any easement issued by the federal government would require a two-week waiting period, but this time the Army Corps specifically waived that pause.

So in what will likely be their final attempt to block the mega-infrastructure project, the local Sioux tribes have reached for a new, emergency legal argument: They allege that the pipeline will harm their First Amendment rights. The tribes argue that their religion depends on the purity and sanctity of the water of Lake Oahe, and that the pipeline could permanently harm this water.

“The Lakota people believe that the pipeline correlates with a terrible Black Snake prophesied to come into the Lakota homeland and cause destruction. The Lakota believe that the very existence of the Black Snake under their sacred waters in Lake Oahe will unbalance and desecrate the water and render it impossible for the Lakota to use that water in their Inipi ceremony,” says a brief filed by the Cheyenne River Sioux early Thursday morning.

As the Army Corps has previously considered moving the pipeline, the Sioux say that it is not an onerous request for them to move it now, out of religious concerns. “The Tribe does not argue that Dakota Access may not have its pipeline elsewhere,” says the brief, “only that this pipeline, sited through these sacred waters, owned in trust by the United States for this tribe, violate the Cheyenne River Sioux Tribe’s and its members’ right to exercise their religion.” (Emphasis is the tribe’s.)

It is invoking the 1993 Religious Freedom Restoration Act to make this claim: a law passed by Congress nearly unanimously under President Bill Clinton that finds religions deserve protection from federal laws that inhibit their adherents from religious practice—even if the law was passed with a non-discriminatory intent.  

Many readers will recognize the Religious Freedom Restoration Act, known as RFRA, for the role that it has played in recent debates over health care and marriage equality.

A 2014 Supreme Court case, Burwell v. Hobby Lobby, determined that the government could not require privately held corporations to cover employees’ contraception if executives had religious objections to doing so. At the state level, many legislatures, including Indiana’s, have proposed RFRA-like legislation in recent years, largely designed to protect private business owners who do not want to provide services at same-sex weddings.

But RFRA originally emerged out of concern for the sovereignty of Native American tribes and nations. In 1990, the Supreme Court ruled that Oregon was within its rights to fire two Native American employees after they failed a drug test for mescaline. Since peyote, which contains mescaline, is critical both to many Native religions and the Christianity-infused Native American Church, the case seemed implicitly to run afoul of freedom of religion—even if it did not explicitly violate the First Amendment.

This ruling, combined with other cases where Native groups alleged that federal infrastructure projects violated sacred land, prompted the initial passage of RFRA.

Yet experts on the law—and the two decades of jurisprudence about it—said that this final attempt from the Sioux tribes were unlikely to stop the pipeline.

“This is a Hail Mary pass,” says Douglas Laycock, a professor at the University of Virginia School of Law. “I don’t think the RFRA claim adds much.”

The problem he identifies comes back to an old, familiar one for the tribe: While the pipeline crosses land that was granted to the Sioux under the 1851 Treaty of Fort Laramie, the land was later stolen from the tribe, and the tribe can make no modern-day property claim. Lake Oahe, meanwhile, is owned by the federal government—that’s why Energy Transfer Partners needs a federal permit. Laycock argues that if the tribe can’t make a property claim, it can’t easily make a RFRA claim.

“If the pipeline burdens the way they use their own property for religious purposes, they might have a RFRA claim, but then they would also have a property-rights claim. If the pipeline is entirely on someone else’s property, they have no right to insist that the use of that other person’s property be restricted in ways important to their religion,” he says.

The tribe threads this needle by saying that the waters of Lake Oahe are held by the federal government in trust to the tribe. “The United States’s trust duty applies specifically to protection of tribal reserved water rights from upstream contamination,” says the brief. Were the pipeline to leak, the United States would abrogate this duty.

The brief makes one more important argument. Under Hobby Lobby, the courts have to be careful not to inquire too much into the sincerity of any religious claim. “Courts must not presume to determine the place of a particular belief in a religion or the plausibility of a religious claim,” wrote Justice Samuel Alito in the majority opinion.

This would strengthen the robustness of the Sioux’s argument. But Laycock said it is unlikely to matter.

“That language in Hobby Lobby is somewhat ambiguous, largely unnecessary to the decision, and seriously ill-considered,” he said. “With this case having a very different political valence from the contraception litigation, we can expect judges to begin to limit the excessive language in Hobby Lobby.”

In its quest to block the Dakota Access pipeline, the Standing Rock Sioux Tribe has reached for one federal statute after another. Over the summer, it argued the pipeline violated its rights to a historic review under the National Historic Preservation Act. Throughout the fall, it sought an environmental-review statement required under the National Environmental Policy Act.

The Obama administration granted it just such a review in early December. But that decision was vacated in the first days of the Trump administration. On Thursday, a former Cabinet secretary to President Obama condemned this decision.

“The Corps’s decision willfully ignores the government’s trust and treaty obligations to tribal nations and the spirit and letter of the law,” said Sally Jewell, a former secretary of the interior under the Obama administration, in a statement. “The tribe is right to pursue legal action. As a citizen, I add my voice to the thousands calling on the Corps to do the right thing and keep their word to the Standing Rock Sioux Nation.”

But so far, the district court judge who heard the tribe’s other lawsuits has found the government’s treaty or trust obligations insufficient to halt the pipeline. And therein lies the final problem for the Standing Rock Sioux. At least under this administration, it doesn’t seem that historical justice will play a role in decisions to grant new permits.

“There is also much rhetoric in the brief about how the government took most of the Lakota’s land by foul means,” said Laycock. “No doubt that is true, but the courts are not going to undo nineteenth-century land transactions at this point.”