Demonstrators stand next to burning tires as armed soldiers and law enforcement officers assemble on Thursday, Oct. 27 to force Dakota Access pipeline protesters off private land where they had camped to block construction. (Mike Mccleary/AP)

Why are celebrities like actor Mark Ruffalo, civil rights leader Jesse Jackson, liberal television anchor Amy Goodman and scores of Native Americans from the Standing Rock Sioux tribe all converging on a isolated spot along the Missouri river of North Dakota?

To block the construction of an oil pipeline. But there’s much more to it than that.

On Thursday, the group was met by scores of police wearing riot gear and riding in military-style armored vehicles. By the end of the day, after firing tear gas, dismantling teepees and seeking to disperse the crowd, police said they had arrested 141 people, according to news reports.

Here’s what’s behind the burning tires and riot shields.

What is this pipeline project?

Civil rights activist Jesse Jackson sits atop a horse Oct. 26, while visiting the protest camp against the Dakota Access oil pipeline outside Cannon Ball, N.D. (James Macpherson/AP)

The segment of pipe near the protest site is just part of the Dakota Access Pipeline Project, a new 1,172-mile, 30-inch diameter $3.7 billion pipeline that will carry high quality crude oil from the rapidly expanding Bakken and Three Forks basins in North Dakota to existing pipeline networks in Patoka, Ill. and refineries in the Midwest, East Coast, Texas and abroad.

Who wins — or loses — from building it?

The pipeline would save money for oil producers in North Dakota, many of whom ship their crude oil in rail cars across Canada and the northern United States. The Dakota Access pipeline could transport as much as 570,000 barrels per day – representing approximately half of Bakken current daily crude oil production.

The reduction in oil transported by rail would also bring down rates for transporting grain out of South Dakota. The competition for transportation has driven up the cost of shipping corn as much as $1 a bushel.

The losers? Railroads that, with little competition, have been charging hefty rates. They include companies such as Burlington Northern Santa Fe, owned by Warren Buffett’s Berkshire Hathaway.

The winners would also include the Dakota Access owners: a partnership called Energy Transfer, and its investors.

They include: Donald Trump. His financial disclosure forms in May said he owned between $500,000 and $1 million of the company. Energy Transfer’s chief executive Kelcy L. Warren this year has made $1.53 million in campaign contributions to Super PACS and $246,900 to individual campaigns and the GOP, according to the Center on Responsive Politics. In June 2015, he gave $5 million to Opportunity and Freedom PAC, which supported Rick Perry’s presidential campaign. The Trump Victory Fund, a joint fundraising committee that includes Trump’s campaign, the Republican National Committee and some state parties, received a $100,000 contribution from Warren on June 29.

Who or what is Energy Transfer?

Energy Transfer is a master limited partnership, a type of corporation that does not pay corporate taxes but which passes profits through to the partners, who pay income tax. (Real estate is another business that often uses limited partnerships to avoid corporate taxes on top of individual income taxes.)

As the protests have intensified, the company shares have dropped from $42.85 on Aug. 8 to $35.42 on Friday.

Why do people oppose the pipeline?

Lots of reasons.

The biggest opponents are environment groups who have built a “Keep It in the Ground” movement that favors throwing up obstacles to companies seeking to extract oil, natural gas or coal.

“We do not want, nor do we need, an expansion of fossil fuel infrastructure,” Sierra Club executive director Michael Brune said in a blog post. “We have the technology and ability to continue and complete our transition to 100 percent clean renewable energy and leave dirty fuels like Bakken crude oil where they belong -- in the ground.”

Another chief critic is Bill McKibben, a Middlebury College professor and leader of climate action group 350.org,

“Since the pipeline has won its ‘expedited’ ‘fast-track’ approvals, the climate science has made it very clear that we simply can’t build any more fossil fuel infrastructure,” McKibben said in an email.

He said the was “no way” the pipeline “could pass the ‘climate test’ that [Hillary] Clinton has promised in her new platform--it clearly would ‘significantly exacerbate’ global warming.”

He added that “‘Keep it in the ground’ is not ideology; it’s math, and that makes clear that we can’t build this pipeline.”

Then there are the Native American Groups, nursing grievances dating back to the 19th century treaties in which tribes ceded much of the Dakotas to the U.S. government. The protestors say the 1851 and 1868 Fort Laramie treaties left areas west of the Missouri River to the Lakota or Sioux tribes. (Rocker Neil Young even has written a new song about it called “Indian Givers.”)

Energy Transfer says it obtained right of way from private landowners. And the Army Corps of Engineers, which approved the route, says the pipeline crosses half a mile north of the modern day Standing Rock Sioux reservation.

Native American groups have also raised two other objections: danger to water supplies and sacred grounds including Lake Oahe. Many of the Standing Rock Sioux live downstream from the spot where the pipeline would cross under the Missouri River, and they cite dangers of a leak similar to ones in Montana’s Yellowstone River or Michigan’s Kalamazoo River. They also say that the pipeline would dig up sacred burial grounds and historic archaeological artifacts.

“We have witnessed inspiring and brave acts by Native Americans and their allies who are defending and trying to protect their sacred sites and the safety of their sole source of water,” Al Gore said in a statement Oct. 25.

The protestors also say that Energy Transfer moved the route of the pipeline, which originally passed through wealthier areas of the state close to Bismarck.

“This is the clearest example of environmental racism in action that you’d ever want to see,” McKibben said. “They were going to run the pipeline across the Missouri above Bismarck till someone said ‘what if it leaks?’ Following the natural path of 400 years of American history, they said ‘Oh, let’s put it above Standing Rock.’ You don’t like Flint? Meet Standing Rock.”

Why do some people support the pipeline?

Many people support the pipeline, even those who worry about climate change. One reason, they say, is that the pipeline would be safer than trains, which have derailed with oil cargos on several occasions.

Another reason, they say, is that we still need the oil and producers will find ways of getting it to market.

“With oil you can barge it, tanker it or put it on railroad cars or on trucks,” said Frank Frank A. Verrastro, a senior fellow on energy at the Center for Strategic and International Studies. “Typically people figure out another way.”

Often that means sending trains with 100 tanker cars through little communities and cities. “I’m not sure it’s a better solution,” he said. “And it all plays back to the keep it in the ground movement. We cannot afford to keep fossil fuels in the ground for the foreseeable future because there’s no scalable replacement. I think the direction is correct but we can’t stop this tomorrow.”

What happens next?

A lot depends on who becomes president.

In September, U.S. District Judge James E. Boasberg said that while he was “aware of the indignities visited upon the Tribe over the last centuries,” he could not grant them an injunction against the construction. He said concerns about sacred grounds could be addressed through the use of tribal monitors or laying the pipeline well below areas with artifacts.

On Thursday, Hillary Clinton’s campaign weighed in on the pipeline.

“Now, all of the parties involved — including the federal government, the pipeline company and contractors, the state of North Dakota, and the tribes — need to find a path forward that serves the broadest public interest,” campaign spokeswoman Xochitl Hinojosa said in a statement.

Height Securities, an investment advisory firm, said “We view this measured response as a positive sign that the candidate is unwilling to bend to environmentalists’ radical demands that the U.S. government invalidate permits it granted in July. Moving forward, we believe that continued consultations between the project and the Standing Rock Sioux tribe are the best option for reaching a timely conclusion to this conflict.”

McKibben had a different interpretation of the Clinton statement. “I think the technical term for it is...pap,” he said. “It literally says nothing.”