From the start of his campaign, after he descended the golden escalator to give his announcement speech, Donald Trump promised to build a wall along the U.S.' Southern border. Now, Trump is taking the first steps toward keeping that promise, with an executive action that calls for building that wall.

In line with his campaign theme of tightening laws on immigration, that action will call for other measures, such as hiring more Border Patrol agents and expanding detention space.

Here's a quick primer on Trump's wall — how long it would have to be, how much it will cost and what Americans think about it.

The situation right now

Trump is no stranger to flip-flopping, and he has voiced different opinions on how he wants the border barrier to look. When CBS's Lesley Stahl asked him if he would "accept a fence," he answered, "For certain areas I would, but certain areas, a wall is more appropriate."

Then at his Jan. 11 press conference, he took a harder line: "On the fence, it's not a fence; it's the wall," he told reporters. "You just misreported it. We're going to build a wall."

Remember, for the record, he said — "For certain areas I would."

Should he decide to accept some fencing, he has existing fencing to add onto: As of late 2015, there were 652.7 miles of border fence along the roughly 2,000-mile-long border.

Along with that head start on the barrier, he also appears to have a head start on the legislative side. When Republican Rep. Luke Messer recently claimed that the fence is authorized, and that it only needs to be funded, PolitiFact ruled it "True." A November 2016 report from the nonpartisan Congressional Research Service bolsters that finding:

"DHS's current policy not to deploy a substantial amount of additional fencing, beyond what is expressly required by law, appears primarily premised on policy considerations and funding constraints, rather than significant legal impediments," writes the author (whose name was redacted from the report).

Still, there are other logistical hurdles. For example, there's the pesky issue of private land ownership. When the U.S. government was building the fence from 2008 to 2010, the Border Patrol "had to enter into negotiations or begin condemnation proceedings with landowners," as NPR's John Burnett reported in mid-2014. And even then, years after the construction, some lawsuits were still ongoing.

Another potential legal barrier exists in a 47-year-old treaty, as the AP reported on Wednesday:

"The Trump administration also must adhere to a decades-old border treaty with Mexico that limits where and how structures can be built along the border. The 1970 treaty requires that structures cannot disrupt the flow of the rivers, which define the U.S.-Mexico border along Texas and 24 miles in Arizona, according to The International Boundary and Water Commission, a joint U.S.-Mexican agency that administers the treaty."

The cost: high. An actual wall could cost $15 billion to $40 billion

A member of President Trump's own party criticized the cost of a wall on Wednesday. Texas Republican Rep. William Hurd, who represents a district featuring a lengthy border with Mexico, said in a statement, "The facts have not changed. Building a wall is the most expensive and least effective way to secure the border."

Here are some basic facts on cost: Trump will need to find funding, and he very well may need a lot of it. While Trump at one point estimated the cost of his wall (or wall-fence combination) to be $8 billion — which he later revised to $10 billion to $12 billion — it could easily be much higher.

Were he to, in fact, try a wall, the cost could be much higher. One estimate from MIT found that a 1,000-mile wall would cost anywhere between $27 billion and $40 billion. Another estimate from research and investment firm Bernstein put it at $15 billion to $25 billion.

Of course, a fence could be far cheaper than a wall. The nearly 653 miles of wall built thus far cost around $2.3 billion, the DHS said in 2015 congressional testimony. That comes out to around $3.5 million per mile.

Multiply that out across 1,300 more miles, and it comes to around $4.6 billion. But there's a big caveat here: That could easily be a very conservative estimate. Estimating how much a border fence costs isn't simply a matter of miles times dollars, because some miles of border fence cost far more than others.

A 2009 analysis from the Government Accountability Office found that as of late 2008, pedestrian fencing had cost the government an average $3.9 million per mile — but that costs ranged from $400,000 to $15.1 million per mile. Likewise, vehicle fencing averaged $1 million per mile but ranged from $200,000 to $1.8 million per mile.

And costs vary depending on who's doing the estimation. One 2009 analysis from the Department of Homeland Security estimated that as of late 2008, it would cost $6.5 million per mile for pedestrian fencing — that is, fencing primarily meant to block people attempting to cross the border on foot. Vehicle fencing came out to $1.7 million per mile.

Prices can vary for a number of reasons; for example, the fence is far taller in some places than others, and different materials are used in different spots. Geography also plays a part in determining cost. Along some stretches of the border, like rough mountain ranges, costs could exceed $10 million per mile, as Burnett has reported.

The cost also depends upon who is doing the building. Commercial builders are more expensive than if the U.S. depends upon the military to build the fence.

And then other costs could easily arise — for example, the cost of legal proceedings from all those dealings with landowners mentioned above.

Cost considerations are less of a worry if Mexico will pay for the wall, as Trump has claimed it would. But he is unlikely to be able to make that happen without a massive fight, as Mexican President Enrique Peña Nieto seemed to indicate earlier this month.

"It is evident that we have differences with the new United States government on some issues, such as a wall that Mexico absolutely will not pay for," Peña Nieto said, as quoted by the Guardian. "At no time will we accept anything that goes against our dignity as a country and our dignity as Mexicans."

Most Americans are opposed to the wall

Many Trump supporters were fervent supporters of a border wall (as evidenced by the loud chants of "Build the wall" at his campaign events). However, polls suggest that Americans as a whole aren't thrilled about a border wall. A recent ABC News poll found that only 37 percent of Americans are in favor of building it.

It's not that Americans aren't concerned about immigration, of course; 72 percent of Americans want to "deport undocumented criminals," according to the ABC News poll. Likewise, a Pew Research Center poll from late November and early December found that large majorities of Americans support a variety of immigration policies: 77 percent said it's "somewhat" or "very important" to have stricter enforcement of visa overstays, and 73 percent said the same of preventing unqualified immigrants from receiving government benefits, for example.

Meanwhile, only 39 percent said the same of building a border wall. Indeed, majorities supported other policies that run counter to the Trump immigration agenda, including allowing people who entered the country as children to stay, as well as supporting a path to legal status.

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