Standing Rock Sioux Chairman Dave Archambault II, photographed in the offices of The Washington Post on Feb. 8, 2017. (Thomas Simonetti/The Washington Post)

Standing Rock Sioux Chairman David Archambault II arrived in Washington on Tuesday afternoon expecting to make a last-minute pitch to head off the Dakota Access Pipeline at a scheduled White House meeting the next morning.

But as he walked through Reagan National Airport, he learned on a phone call that he might as well have stayed in North Dakota. The Army Corps of Engineers had decided to grant the company behind the pipeline the critical easement it needed, rendering his meeting with the Trump administration moot.

“I just feel that I was slighted. I was disrespected. I think that I was set up,” said Archambault, whose tribe has been fighting the pipeline since 2014 on the grounds that it would infringe on the tribe’s rights and could pose a risk to its drinking water.

The machinery of the federal government often moves slowly. But in the past two weeks, at President Trump’s urging, a process that his predecessor had decelerated was suddenly moving forward, culminating in the Army’s decision Wednesday to give Energy Transfer Partners an easement to drill under a vast reservoir less than a mile from the Standing Rock Sioux’s reservation.

Army Corps of Engineers officials had been poised late last year to grant the easement under Lake Oahe, clearing the way for completion of a pipeline that would transport oil from North Dakota to a network of pipelines and refineries in Illinois. But officials in three other agencies — Interior, Justice and the Environmental Protection Agency — raised concerns about whether there had been adequate consultation with the tribe and a review of the project’s potential environmental impacts. In December, President Barack Obama instructed the Corps to conduct a full environmental impact statement and to explore alternate routes.

Rep. Kevin Cramer (R-N.D.), a proponent of the pipeline, said Wednesday that the Army studied the issue extensively and had been stymied by the “political pressure” Obama and some of his appointees exerted.

“They’ve been comfortable with this decision for many months,” Cramer said.

Trump said Wednesday that he received no indications that anyone objected to the pipeline’s completion. “I did the Dakota pipeline, and nobody called up to complain,” he said. “I haven’t had one call from anybody.”

But Archambault, who expressed his frustration to a White House official last week and thought he would have a chance to make his case before the critical easement decision, disagrees.

“My intention for meeting anyone in the White House who is willing to listen is to just share why there is a movement, why there is resistance,” he said. “And help them understand all the wrongs that have been committed to my nation by this nation.”

Archambault is not pushing for protesters to again flock to North Dakota the way they did in the final months of 2016. That demonstration “served its purpose,” he said, and there will be more environmental fights.

“I’ve been encouraging people to go to their state capitals and their congressional districts and make that the front,” he said. “We’re going to have a march here in D.C. on March 10. We have to let everyone know that indigenous people are united. . . . We’re not going anywhere.”

When top members of the National Sheriffs’ Association met with Trump in the White House on Tuesday, one of their chief requests was for federal help dealing with the demonstrators at the pipeline. Local law enforcement feels stymied by the protesters’ ability to retreat to reservations — land where law enforcement has no jurisdiction. Sheriff Paul Laney, of Cass County, N.D., said Trump did not commit to additional federal aid; similar requests of the Obama administration were denied.

Former interior secretary Sally Jewell, who has refrained from publicly criticizing the new administration, said in an interview that Corps officials had not fully taken tribal or environmental concerns into account. She added that the agency’s announcement that it intended to conduct a full-scale review, which it withdrew Tuesday, aimed to address that.

“So the decision to not do any of that is reneging on a commitment they made, and I think it’s fair to say that I’m profoundly disappointed with the Corps’ reversal of its decision to conduct an environmental impact statement and consider alternative routes,” Jewell said. “This is a clear reversal of a commitment on the part of the U.S. Army Corps of Engineers on something they gave thoughtful consideration to when they decided to do an environmental review.”

An Army spokesman noted in an email that acting secretary of the Army Robert Speer said Tuesday that he was responding to Trump’s Jan. 24 directive and “the decision was made based on a sufficient amount of information already available which supported approval to grant the easement request.”

Energy Transfer Partners, the pipeline’s sponsor, said Wednesday, “With this action, Dakota Access now has received all federal authorizations necessary to proceed expeditiously to complete construction of the pipeline.”

It said it would complete financial transactions “within the next several days” that would secure the remaining $2.6 billion needed for the project, which it said would be in operation “in the second quarter of 2017.”

Proponents of the pipeline, including the United Association, a union of plumbers, fitters, welders and service techs, said finishing it would translate into immediate employment for men and women on the Great Plains. The UA has had 1,100 members working on the project and will have another 50 help finish it up. Once construction starts, the pipeline could be operational in another 60 to 80 days.

“This action gives the hard-working members of the United Association a chance to earn wages and generate income for local businesses,” the union said in a statement.

Cramer, who said he is friendly with Archambault and recently watched his son play basketball in Bismarck, said he hoped tribal leaders could say to federal officials, “You know, we’re going to chalk this loss up, but let’s talk about how to do this better next time.”

But Archambault does not appear inclined to back down. He canceled his White House meeting on Wednesday, but he said the tribe plans to challenge the easement in court.

“We’re going to continue and do whatever we can,” he said, “even though the options are running out.”

Tom Jackman contributed to this report.