(CNN) With one swipe of the presidential pen, the Dakota Access and Keystone XL pipelines were back on the agenda, marking the realization of protesters' worst fears about what a Donald Trump presidency means for the environment and the controversial energy projects.

Trump signed executive actions Tuesday to advance the approval of the pipelines -- one of which, the Dakota Access, was recently stalled, while the other had been killed by the State Department and White House in 2015.

The proposed $3.7 billion Dakota Access Pipeline was slated to stretch 1,172 miles through four states -- from North Dakota into South Dakota, winding through Iowa and ending in southern Illinois -- moving 470,000 barrels of crude oil a day across the Midwest. It is completed except for a contested portion under North Dakota's Lake Oahe, half a mile upstream from the Standing Rock Sioux tribe's reservation.

The $8 billion Keystone XL Pipeline was proposed to stretch nearly 1,200 miles across Montana, South Dakota, Nebraska, Kansas, Oklahoma and Texas, carrying more than 800,000 barrels of carbon-heavy petroleum daily from Canada's oil sands to refineries on the Gulf Coast.

With debates looming, and protests quickly mobilizing, how did the nation get to this point?

Proponents of both projects tout their economic boon. Supporters say the Dakota Acess pipeline would decrease American reliance on foreign oil. Opponents cite environmental concerns, such as contamination due to breaches and eventual greenhouse gas emissions, and destruction of Native American land and burial sites.

Protest the new administration's reversal on the Dakota Access Pipeline. I wasn't active enough in the effort last time, and the fights going to be harder this time around. #nodapl #waterislife #honorthetreaties A photo posted by Scy (@scyofthesky) on Jan 24, 2017 at 4:07pm PST

History of the Dakota Access Pipeline

The US Army Corps of Engineers approved the project and granted final permits in July to the dismay of environmentalists and the Standing Rock Sioux.

The tribe quickly sued the corps, claiming the pipeline threatens its "environmental and economic well-being, and would damage and destroy sites of great historic, religious, and cultural significance."

The Army Corps of Engineers has declined to comment, citing pending litigation.

Photos: North Dakota pipeline protests Fireworks lit the sky at the Oceti Sakowin Camp near Cannon Ball, North Dakota on December 4, 2016, after the Army Corps of Engineers halted the Dakota Access Pipeline route. An executive order by President Donald Trump in January allowed work to resume. Hide Caption 1 of 32 Photos: North Dakota pipeline protests Activists embrace after the December halt of the Dakota Access Pipeline route. The $3.7 billion project that would cross four states and change the landscape of the US crude oil supply. The Standing Rock Sioux tribe says the pipeline would affect its drinking-water supply and destroy its sacred sites. Hide Caption 2 of 32 Photos: North Dakota pipeline protests An activist rides down from a ridge on the edge of the Standing Rock Sioux Reservation on December 4. Hide Caption 3 of 32 Photos: North Dakota pipeline protests Activists celebrate at Oceti Sakowin Camp on December 4. An executive order by President Donald Trump in January allows work to resume on the Dakota Access Pipeline, which the activists oppose. Hide Caption 4 of 32 Photos: North Dakota pipeline protests Dan Nanamkin of the Colville Nez Perce tribe drums a traditional song by the Cannonball River in Cannon Ball, North Dakota, on Thursday, December 1. Hide Caption 5 of 32 Photos: North Dakota pipeline protests A procession makes its way down to the Cannonball River to take part in a Native American water ceremony on December 1. Hide Caption 6 of 32 Photos: North Dakota pipeline protests Snow covers the camp on Wednesday, November 30. Hide Caption 7 of 32 Photos: North Dakota pipeline protests A person walks through snow and wind on Tuesday, November 29. Hide Caption 8 of 32 Photos: North Dakota pipeline protests People against the construction of the Dakota Access Pipeline speak at a news conference near Cannon Ball on Saturday, November 26. Hide Caption 9 of 32 Photos: North Dakota pipeline protests A sculpture stands at an encampment where protesters of the pipeline have been gathered for months. Hide Caption 10 of 32 Photos: North Dakota pipeline protests People protest along Highway 1806 as they walk past a sprawling encampment on Thursday, November 24. Hide Caption 11 of 32 Photos: North Dakota pipeline protests A man stands along Highway 1806 on November 24. Hide Caption 12 of 32 Photos: North Dakota pipeline protests In this image provided by the Morton County Sheriff's Department, law enforcement and protesters clash near the pipeline site on Sunday, November 20. Hide Caption 13 of 32 Photos: North Dakota pipeline protests Tonya Stands recovers after being pepper-sprayed by police on Wednesday, November 2. Stands was pepper-sprayed after swimming across a creek with other protesters hoping to build a new camp to block construction of the Dakota Access Pipeline. Hide Caption 14 of 32 Photos: North Dakota pipeline protests Dozens of protesters wade in cold creek waters as they confront local police on November 2. Hide Caption 15 of 32 Photos: North Dakota pipeline protests Tribe members make their way back to their camp on Saturday, October 29. Hide Caption 16 of 32 Photos: North Dakota pipeline protests Cousins Jessica and Michelle Decoteau take part in a protest outside the North Dakota state capitol in Bismarck on October 29. Hide Caption 17 of 32 Photos: North Dakota pipeline protests The burned hulks of heavy trucks sit on Highway 1806 on Friday, October 28, near a spot where Dakota Access Pipeline protesters were evicted a day earlier. Hide Caption 18 of 32 Photos: North Dakota pipeline protests Pipeline protesters sit in a prayer circle as a line of law enforcement officers make their way across the camp to relocate the protesters a few miles south on Thursday, October 27. Protesters had camped on private property. Hide Caption 19 of 32 Photos: North Dakota pipeline protests A protester is arrested as law enforcement surrounds the camp on October 27. Hide Caption 20 of 32 Photos: North Dakota pipeline protests Tires burn as armed soldiers and law enforcement officers stand in formation to force Dakota Access Pipeline protesters off the private land in Morton County. Hide Caption 21 of 32 Photos: North Dakota pipeline protests A protester shows where he was hit by a bean-bag round fired by officers trying to force protesters off the private land. Hide Caption 22 of 32 Photos: North Dakota pipeline protests JR American Horse leads a march to the pipeline site on Friday, September 9. Hide Caption 23 of 32 Photos: North Dakota pipeline protests Native Americans head to a rally at the state capitol in Denver on Thursday, September 8. They were showing their support for members of the Standing Rock Sioux tribe in North Dakota opposting the Dakota Access Pipeline. Hide Caption 24 of 32 Photos: North Dakota pipeline protests People hang a sign near what they say was sacred burial ground disturbed by bulldozers in Cannon Ball. Hide Caption 25 of 32 Photos: North Dakota pipeline protests Marlo Langdeau of the Lower Brule Sioux Tribe joins hundreds of Native Americans for a march near Cannon Ball on Sunday, September 4. Hide Caption 26 of 32 Photos: North Dakota pipeline protests Protesters march on September 4 in opposition to the Dakota Access Pipeline. Hide Caption 27 of 32 Photos: North Dakota pipeline protests Native Americans march to the site of a sacred burial ground on September 4. Hide Caption 28 of 32 Photos: North Dakota pipeline protests Native Americans ride with raised fists to the sacred burial ground on September 4 to protest the Dakota Access Pipeline. Hide Caption 29 of 32 Photos: North Dakota pipeline protests The Missouri River is seen beyond an encampment near Cannon Ball, where hundreds of people gathered to join the protest on September 4. Hide Caption 30 of 32 Photos: North Dakota pipeline protests Flags of Native American tribes from across the United States and Canada line the entrance to a protest encampment on Saturday, September 3. Hide Caption 31 of 32 Photos: North Dakota pipeline protests Phil Little Thunder Sr. attends an evening gathering at an encampment of Dakota Access Pipeline protesters on September 3. Hide Caption 32 of 32

What supporters claimed

The developer, Dakota Access , a subsidiary of Energy Transfer Crude Oil, said the pipeline from the oil-rich Bakken Formation, a vast underground deposit where Montana and North Dakota meet Canada, is the most cost-effective and environmentally responsible way to move crude oil, removing the dependency on rails and trucks.

An estimated 7.4 billion barrels of undiscovered oil is believed to be in the US portion of the Bakken, according to the US Geological Survey

Dakota Access estimates the pipeline would generate $156 million in sales and income taxes to state and local governments as well as add 8,000 to 12,000 construction jobs.

An advocacy group, the Midwest Alliance for Infrastructure Now , backed the developer's assertions that pipelines are a safe way to move crude oil.

"Already, eight pipelines cross the Missouri River carrying hundreds of thousands of barrels of energy products every day," the group said.

The alliance said the pipeline "does not cross into the Standing Rock Sioux tribe's reservation."

The alliance also said 100% of the affected landowners in North Dakota , where part of the tribe lives, voluntarily signed easements allowing construction.

What opponents argued

The Standing Rock Sioux tribe is also concerned that digging the pipeline under the Missouri River would affect the area's drinking water.

"We have laws that require federal agencies to consider environmental risks and protection of Indian historic and sacred sites," Dave Archambault II, the elected chairman of the Standing Rock Sioux tribe, said in an earlier statement

"But the Army Corps has ignored all those laws and fast-tracked this massive project just to meet the pipeline's aggressive construction schedule."

Based in Fort Yates, North Dakota, Standing Rock is a federally recognized Indian tribe, a successor to the Great Sioux Nation. Other Native American tribes and nations have joined in opposition.

Archambault II said he doesn't support moving more crude oil from North Dakota. He told CNN affiliate KFYR-TV in Bismarck the country should search for alternative and renewable sources of energy.

What happened last

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For months, Standing Rock Sioux members and allies protested in North Dakota. They stood in the path of the pipeline during peaceful demonstrations and clashes that turned violent. Police deployed bean bag rounds and pepper spray and unleashed a high-pitched siren to disperse the crowd.

One day in October, police arrested at least 141 people. A few weeks later, just before Thanksgiving, as temperatures dipped below freezing, authorities unleashed water cannons on the demonstrators.

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In her letter to the Army Corps, Jo-Ellen Darcy, the Army's assistant secretary for civil works, called for the creation of an official environmental impact statement , a months-long process that would allow the public to weigh in.

The Army statement did not rule out future approval of the current route. But tribal leaders worried then that the decision to change direction might not be permanent, especially with the incoming Trump administration and pipeline supporters backing the plan.

This month, a judge blocked an effort by Dakota Access to stop the Army Corps from beginning the environmental impact study.

Supporters of the project praised Trump's decision Tuesday.

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"This is clearly a step in the right direction," North Dakota Public Service Commissioner Julie Fedorchak told KFYR.

The Standing Rock Sioux tribe vowed to keep fighting.

"It wasn't a surprise. We knew this was gonna happen. We've been preparing for it," Jon Eagle Sr., Standing Rock's historic preservation officer, told KFYR . "You gotta take a historic perspective, though, of who we are as Lakota Dakota people. We've been resisting since the point of contact."

History of Keystone XL

TransCanada first applied for a permit to build the pipeline in 2008. Since then, the massive project has been a hot topic during presidential and congressional elections. It pitted oil companies and Republicans against environmentalists and liberal activists.

Keystone XL represented just under a third of TransCanada's entire Keystone project. Every other piece of the project had been built and laid out.

What supporters say

The Canadian and, in particular, Alberta government pushed President Barack Obama to approve the project. TransCanada said in 2011 that the project would create about 140,000 direct and indirect jobs.

The State Department in January 2015 concluded the project would create about 42,000 jobs directly and indirectly. The total included about 3,900 construction jobs.

In all, the pipeline would provide about $2 billion in economic benefits, the State Department review said. But once the construction jobs, which would last no more than two years, wrapped up, Keystone XL would have created about 50 permanent jobs, the number needed to maintain the pipeline.

What opponents argued

Environmentalists were concerned because extracting crude oil from the oil sands, a combination of oil and sand, pumps about 17% more greenhouse gases into the air than standard crude oil extraction, according to the State Department review.

Environmentalists, local residents and indigenous tribes also said the pipeline's proposed route would cut across the Ogallala Aquifer, which is one of the world's largest. Activists were also concerned the pipeline could pollute many of the 2,500 aquifers, key sources of fresh water, within a mile of the proposed route.

Native Americans voiced concerns over the societal impact of camps with thousands of construction workers living near their communities. Those face high rates of sexual assaults from nonindigenous men, representatives of the activist Wica Agli group said in 2015.

What happened last

In 2011, the Obama administration said it would delay a decision on the pipeline until at least 2013 to allow for more time to study the issues and to look at possible alternative routes.

Exercising his veto power for the first time in five years , Obama rejected legislation giving the go-ahead for construction of the pipeline in February 2015.

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The White House said it opposed the bill because it would have usurped the Obama's authority to approve or deny the creation of the pipeline and short-circuit the State Department analysis.

In November 2015, Obama nixed the proposed pipeline , virtually ending the fight over the project that had gone on for much of his presidency.

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Secretary of State John Kerry concluded the project was not in the country's national security interest.

Obama's move came as the White House continued to promote his environmental agenda and efforts to fight climate change.

In a statement at the time, Kerry said the climate impact was the key factor.

"The critical factor in my determination was this: Moving forward with this project would significantly undermine our ability to continue leading the world in combating climate change," he said.

The pipeline was a key issue in the 2012 presidential campaign, with GOP nominee Mitt Romney saying he would approve the pipeline.

Republican candidates in the 2016 race also pledged to let the project go forward.

At the time, Trump, then the GOP front-runner, tweeted: "So sad that Obama rejected Keystone Pipeline. Thousands of jobs, good for the environment, no downside!"

So sad that Obama rejected Keystone Pipeline. Thousands of jobs, good for the environment, no downside! — Donald J. Trump (@realDonaldTrump) November 6, 2015

Canadian Prime Minister Justin Trudeau, who has said he was "disappointed by the decision," expressed support for the pipeline Tuesday.

"I've been on the record for many years supporting it because it leads to economic growth, and good jobs for Albertans," he said.

In a statement, Sierra Club Executive Director Michael Brune said, "The Keystone pipeline was rejected because it was not in the country's interest, and the environmental review of the Dakota Access Pipeline was ordered because of the threats it poses to the Standing Rock Sioux. Nothing has changed."