(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 on Tuesday signed executive actions to advance the approval of the stalled pipelines.
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 -- and move 470,000 barrels of crude oil a day across the Midwest. It is completed except for a contested portion under Lake Oahe in North Dakota, which is 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 six states -- Montana, South Dakota, Nebraska, Kansas, Oklahoma and Texas -- and carry more than 800,000 barrels of carbon-heavy petroleum daily from Canada's oil sands through Nebraska to refineries on the Gulf Coast.
With debates looming, and protests quickly mobilizing, like those on Tuesday in Washington, New York and Seattle, how did the nation get to this point with the controversial pipelines?
Proponents of both projects tout their economic boon. Supporters say the Dakota 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.
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 tribe.
The Standing Rock Sioux quickly sued the corps, claiming the pipeline "threatens the tribe's environmental and economic well-being, and would damage and destroy sites of great historic, religious, and cultural significance to the tribe."
The Army Corps of Engineers has declined to comment, citing pending litigation.
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 claims that pipelines are a safe way of moving 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 the country should search for alternative and renewable sources of energy.
What happened last
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.
In one day, police arrested at least 141 people.
But in December, protesters celebrated after the Army said it would not -- for the time being -- allow the pipeline to cross under Lake Oahe. The Army said the plan should be carefully restudied, and alternative routes should be deeply considered.
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 monthslong 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 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 block the Army Corps from starting the environmental impact study.
Supporters of the project praised Trump's decision on Tuesday.
"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 Alberta governments, in particular, pushed former President Obama to approve the project. TransCanda 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 to build the pipeline.
In all, the pipeline would provide about $2 billion in total 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 about 17% more greenhouse gases into the air than standard crude oil extraction, according to a State Department review of the project.
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 freshwater, 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 non-indigenous 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.
The White House said then it opposed the bill because it would have usurped the President'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.
Then-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."
The pipeline was a key issue in the 2012 presidential campaign, when GOP candidate Mitt Romney said 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 frontrunner, tweeted: "So sad that Obama rejected Keystone Pipeline. Thousands of jobs, good for the environment, no downside!"
Canadian Prime Minister Justin Trudeau, who said then he was "disappointed by the decision," expressed support for the pipeline on 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."
CNN's Melanie Whitley and Holly Yan contributed to this report.
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