Trump says border wall will 'also help Mexico'

President Trump on Wednesday took the first step toward fulfilling his campaign promise of building a "great wall" along the southwest border. Getting it finished, however, will take a lot of help from Congress.

During a speech at the Department of Homeland Security, Trump said his administration would "begin immediate construction of a border wall," a campaign pledge so popular with his supporters that they chanted "Build that wall" at rallies. Earlier in the day, Trump told ABC News that the planning of the wall had begun, but construction could would start in several months.

"So badly needed," Trump told a room full of DHS employees. "This will also help Mexico by deterring illegal immigration from Central America and by disrupting violent cartel networks."

Here's a look at some of the basics behind the border wall.

Where does it stand now?

The U.S. "border wall" is really a collection of walls, fences and other barriers that currently cover 652 miles of the 1,954-mile border with Mexico, according to the Government Accountability Office.

The wall is strongest along parts of the border that have large populations on both sides, such as San Diego and Tijuana. Each city runs right up to the border, so the Department of Homeland Security has built double and triple layers of fencing for 14 miles starting at the Pacific Ocean and heading east.

In other, more desolate parts of the border, there are only single fences or a variety of barriers, including iron vehicle barriers that people can easily walk past. Most areas have no fencing, but some are nearly impossible to cross, from the widest sections of the Rio Grande to deep canyons and treacherous mountains.

How much could it cost to finish?

Predictions have varied widely, but it will be expensive.

Trump took his own stab at the figure in February, telling MSNBC he could finish the wall for $8 billion. But most other estimates are far higher.

In 2015, Border Patrol officials said during a Senate hearing that the current 652 miles of wall had cost $2.3 billion. But that does not include increased costs for building across more difficult terrain or building the wall higher, as Trump has said he would do.

The Bernstein Research group conducted a study in July and estimated the total costs to be between $15 and $25 billion.

Could Trump finish the wall himself?

It's possible that Trump could kick-start some construction on his own.

When Congress passed the Secure Fence Act of 2000, it approved up to 700 miles of fencing. With 652 miles currently built, that means Trump could try and build the remaining miles on his own.

"At this point, his goal was to get the project started as quickly as possible using existing funds and resources that the department currently has," Spicer said Wednesday.

But to build the additional 1,200 miles, he would need funding from Congress. White House Press Secretary Sean Spicer said Wednesday that the administration is already working with Congress on an "appropriation schedule" to secure funding for the remainder of the wall.

GOP leaders in both chambers have said they would help Trump secure the border, but have not committed to funding the billions Trump needs.

Can he make Mexico pay for it?

Trump has repeatedly said that he would get Mexico to pay for the construction of the wall.

"Absolutely, 100%" he told ABC News.

Former Mexican President Vicente Fox Quesada took to Twitter again Wednesday to say the country won't pay for that "expletive" wall.

Trump's plan calls for Mexico to pay through a combination of factors, including increased fees and taxes to cross the southwest border, and withholding remittances that Mexican nationals in the U.S. send back to their families in Mexico. He could need some help from Congress there to create a legal mechanism to withhold that much money. The plan could also face legal challenges.

Trump introduced a new possibility on Wednesday. An executive order he signed instructed all executive branch agencies to identify all federal assistance they sent to Mexico over the past five years. That includes all "bilateral and multinational development aid, economic assistance, humanitarian aid and military aid."

The agencies are required to submit a report within 30 days outlining those funds, which could be used as leverage to get Mexico to help pay for the border wall.