BISMARCK, N.D. (AP) – The U.S. 1,200-mile pipeline through the Dakotas, Iowa and Illinois will be safe and that the Corps’ decision is politically motivated.

Here’s a guide to the latest developments and key background about the protest:

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EASEMENT DENIAL

The Corps on Sunday denied an easement for a pipeline crossing beneath Lake Oahe, a Missouri River reservoir and the source of drinking water for the nearby Standing Rock Reservation. Assistant Secretary for Civil Works Jo-Ellen Darcy said her decision was based on the need to consider alternative routes.

Protesters who have been camped near the pipeline route for months have praised the decision. North Dakota leaders including Gov. Jack Dalrymple criticized it.

Energy Transfer Partners, of Dallas, said the decision is politically motivated and that President Barack Obama’s administration is just delaying the matter until he leaves office.

Protesters, who describe themselves as “water protectors,” say they have no plans to leave despite the Corps’ decision, with President-elect Donald Trump, a pipeline supporter, set to take office in January.

The company last month asked a federal judge to allow it to move ahead with the river crossing. A decision is pending.

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EVICTION NOTICE

The Corps said in a letter toliving protest against the $3.8 billion pipeline.

The Corps cited North Dakota’s oncoming winter and contentious clashes between protesters and police. The agency says “it has no plans for forcible removal,” but anyone on land north of the river after the deadline will be trespassing and may be prosecuted.

The agency previously said it would not evict protesters because they are exercising their right to free speech.

North Dakota officials contend the protesters need a permit. Dalrymple last week issued a “mandatory evacuation” for the main encampment but said no one would be forcibly removed.

More than 550 people from throughout the country have been arrested since August. In a late-November clash between police and protesters near the path of the pipeline, officers used tear gas, rubber bullets and large water hoses in freezing weather. Organizers said at least 17 protesters were taken to the hospital, some for hypothermia and one with a serious arm injury. One officer was injured.

North Dakota officials say law enforcement at the protest site has cost the state about $20 million.

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DISPUTED LAND

The Oceti Sakowin camp is the overflow from smaller private and permitted protest sites nearby. It began to grow in August and has been called the largest gathering of Native American tribes in a century.

Standing Rock tribal members and other Native Americans believe the land with the encampment is rightly owned by the Sioux through a more than century-old treaty with the U.S. government.

Protest organizers said they don’t intend to leave or stop their acts of civil disobedience.

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THE ORIGINS

Energy Transfer Partners got federal permits for the pipeline in July, about two years after it was announced. The pipeline is projected to move a half-million barrels of crude oil daily to an existing pipeline in Patoka, Illinois, for shipment to Midwest and Gulf Coast markets.

Supporters say the pipeline will have safeguards against leaks, and is a safer way to move oil than trucks and trains, especially after a handful of fiery – and sometimes deadly – derailments of trains carrying North Dakota crude.

The Standing Rock Sioux, whose reservation straddles the North Dakota-South Dakota border, are suing federal regulators for approving the pipeline. They have challenged the Corps’ decision to grant permits at more than 200 water crossings and argue that the pipeline less than a mile upstream of the reservation could affect drinking water for more than 8,000 tribal members and millions downstream.

The tribe hasn’t fared well in court. A federal judge in September denied its request to block construction of the entire pipeline. An appeal is pending.

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MISSING LINK

The pipeline is largely complete except for the section that would pump oil under Lake Oahe.

The Corps of Engineers granted Energy Transfer Partners the permits needed for the crossing in July, but the agency decided in September that further analysis was warranted given the tribe’s concerns, leading to Sunday’s decision.

The company responded by asking U.S. District Judge James Boasberg to declare that it has the right to lay pipe under Lake Oahe. The judge isn’t likely to issue a decision until January at the earliest.

Obama raised the possibility of rerouting the pipeline in early November, and Archambault has said that would be acceptable to the tribe provided the new route isn’t near the reservation. But Energy Transfer Partners CEO Kelcy Warren told The Associated Press that that the pipeline won’t be rerouted and the company has no alternative than to stick to its plan.

Trump holds stock in Energy Transfer Partners. Pipeline opponents worry that Trump’s investments could affect any decision he makes on the project as president.