Last month, hundreds clashed with law enforcement officials who used water to repel protesters in below-freezing weather, attracting more attention to the conflict. During that confrontation, Sophia Wilansky, 21, who grew up in the Bronx, suffered one of the most serious injuries of the movement, her arm badly damaged from an explosion whose origin remains in dispute. As many as 2,000 veterans plan to join the demonstrators next week to serve as “human shields” against what they describe as a “militarized police force.”

In all, hundreds of people have been arrested since the protests began to attract widespread attention late this summer.

What does each side want?

The Dakota Access pipeline is a $3.7 billion project that would carry 470,000 barrels of oil a day from the oil fields of western North Dakota to Illinois, where it would be linked with other pipelines. Energy Transfer says the pipeline will pump millions of dollars into local economies and create 8,000 to 12,000 construction jobs — though far fewer permanent jobs to maintain and monitor the pipeline.

Members of the Standing Rock Sioux Tribe see the pipeline as a major environmental and cultural threat. They say its route traverses ancestral lands — which are not part of the reservation — where their forebears hunted, fished and were buried. They say historical and cultural reviews of the land where the pipeline will be buried were inadequate. They also worry about catastrophic environmental damage if the pipeline were to break near where it crosses under the Missouri River.