BISMARCK, N.D. (AP) — The U.S. Army Corps of Engineers and two Native American tribes that oppose the $3.8 billion Dakota Access oil pipeline have asked a federal judge to reject an effort by the four-state project's Texas-based developer to get permission to finish it.

It's the latest development in a saga that has played out over the past half-year, with months of protests in southern North Dakota and two separate but related court battles in U.S. District Court in Washington, D.C.

Here's a look at the status of the pipeline dispute as the new year begins:

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WHAT IS THE DAKOTA ACCESS PIPELINE AND WHAT'S THE FUSS ABOUT?

Being developed by Energy Transfer Partners, the project will transport North Dakota oil 1,200 miles through South Dakota and Iowa to a shipping point in Illinois. Construction is nearly complete outside of a stretch under Lake Oahe, a Missouri River reservoir in southern North Dakota, where the work is stalled.

The Standing Rock and Cheyenne River Sioux tribes are fighting the pipeline because the tribes believe it threatens drinking water — both use the lake for that — and cultural sites. ETP disputes that and maintains the pipeline will be safe.

Opponents have protested for months in southern North Dakota, with nearly 600 arrests since August.

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SO, EXPLAIN THE COURT BATTLES ...

There are two cases.

In one, ETP asked U.S. District Judge James Boasberg in November to declare it has legal right to lay pipe under Lake Oahe. ETP subsidiary Dakota Access LLC argues that the Army Corps of Engineers gave permission in July through a permit granted under the federal Rivers and Harbors Act, which safeguards public works projects, such as the dam-created reservoir system.

The Corps said in its response filed Friday that provisions of the federal Mineral Leasing Act also must be satisfied for ETP to drill under the lake. Those provisions include approval of an easement to work on federal land and the notification of Congress, neither of which has been done, agency attorneys said.

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The two tribes also are asking Boasberg to reject ETP's request.

In the other case, the tribes are challenging federal permits for the pipeline at more than 200 water crossings. Last month, the tribes said they were willing to put those claims on hold until the battle between ETP and the Corps is resolved, and Boasberg agreed.

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WHAT'S AT STAKE?

For ETP, it's money. The company had planned to finish the pipeline by the end of 2016. In November, it said delays already cost more than $450 million and anything more would be "tens of millions of dollars each month."

The Sioux tribes maintain what's at stake is clean water for more than 8,000 tribal members and millions of people downstream — as well as Native American cultural sites.

For the Corps, it's the ability to do a more thorough environmental study of the project in the wake of the tribes' concerns. Assistant Army Secretary Jo-Ellen Darcy in a Dec. 4 statement said study is needed on alternative locations for the river crossing, the potential for a leak and tribal treaty rights.

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WHEN WILL IT BE RESOLVED?

A resolution is at least several weeks away under a scheduling order from Boasberg. In the meantime, he's asked that "the government shall promptly notify the court should it change its position regarding the easement." Some pipeline opponents worry that pro-energy President-elect Donald Trump will overturn the Army's easement decision.

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WHAT'S HAPPENING IN NORTH DAKOTA?

No construction work is being done while the court battles play out.

The once-large protest camp near the Standing Rock Reservation — and the pipeline route just to the north — has dwindled to a few hundred people due to the work stoppage and harsh winter weather.

Standing Rock Chairman Dave Archambault last month called for the camp to disband due to dangerous weather and because it's in an area that could flood in the spring, possibly leading to the river being contaminated with garbage and debris. He told The Associated Press late last week that he's unhappy people are still camped out and asked them to devise a plan for leaving the area and cleaning it up.

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