Sheer cost ... and Mexico stand in way of grand vision
Donald Trump is setting out to fulfil one of his key campaign promises, with an executive order to build his proposed wall along the US-Mexico border.
To restrict illegal immigration, Mr Trump has promised to build a wall on the southern US border and to deport illegal migrants living inside the United States. On his personal Twitter account on Tuesday, Mr Trump tweeted: "Big day planned on NATIONAL SECURITY tomorrow. Among many other things, we will build the wall!"
Here's everything you need to know about the planned wall.
When did he propose building the wall?
Mr Trump's presidential campaign got off to an explosive start in June 2015 when he used incendiary language in announcing plans to build a wall along the border with Mexico.
"When Mexico sends its people, they're not sending their best," he said. "They're sending people that have lots of problems... They're bringing drugs. They're bringing crime. They're rapists."
Mr Trump pledged to keep migrants out. "I will build a great, great wall on our southern border, and I will have Mexico pay for that wall. Mark my words," he said.
Despite the outcry over the remarks, Mr Trump's proposal resonated with a large block of voters and it became a pillar of his presidential campaign.
How long will it be?
The US border with Mexico is roughly 3,057km long and spans four states: California, Arizona, New Mexico and Texas. In comparison, the Berlin Wall was 154km, while the Great Wall of China is 20,921km long.
There is already roughly 1,126km of fence along the border - the remainder is either open, nearly impossible to actually build on, or impassable.
Mr Trump said in 2015: "You know, the Great Wall of China, built a long time ago, is 13,000 miles. I mean, you're talking about big stuff. We're talking about peanuts, by comparison, to that."
Mr Trump has said his wall will cover 1,609km, with natural obstacles protecting the rest of the border.
How much will it cost?
Estimates vary hugely. In February last year, Mr Trump told MSNBC he could finish the wall for $8bn (€6.7bn). Most other estimates are far higher.
The existing border fence cost about $2.4bn (€2bn). Building the rest would cost between $15bn (€12.7bn) and $25bn (€21bn), with an annual maintenance cost of $700m (€592m), according to an estimate by Marc Rosenblum, the deputy director of the US Immigration Policy Programme at the non-partisan Migration Policy Institute, who is cited in a report in January last year.
Who will pay for it?
The Mexicans, says Mr Trump. No chance, says Mexico.
During his campaign, Mr Trump boasted he would "make Mexico pay" for the wall by seizing remittances from undocumented immigrants from the country and increasing fees on entry visas to the US for Mexican citizens.
After the Republican candidate met President Enrique Pena Nieto in September, he said the pair had not discussed the issue - a claim contradicted by the Mexican leader. "At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall," Mr Nieto said at the time.
Earlier this month, Mr Trump said the building project would initially be paid for with a congressionally approved spending bill and Mexico would eventually reimburse the US, though he has not specified how he would guarantee payments. Mr Trump will meet Mr Nieto at the White House next week.
Does he need approval from Congress?
No. In claiming authority to build a wall, Mr Trump is expected to rely on a 2006 law that authorised several hundred miles of fencing along the frontier. That bill led to the construction of about 700 miles of various kinds of fencing designed to block both vehicles and pedestrians.
Is it actually a wall?
Mr Trump admitted after his election in November that it may in fact be part wall and part fence. "I'm very good at this, it's called construction," he said at the time.