Trying to purchase this land could be a major challenge and if people refuse, the government would have to forcibly get hold of it.

Welcome to the term "eminent domain".

Eminent domain is a system used to gain ownership of private property for public use, such as for highways and railroads, usually accompanied by compensation. It has been used for the construction of border fences in the past.

Gerald S Dickinson, assistant professor of law at the University of Pittsburgh Law School, has warned that such eminent domain fights could take years.

Any federal eminent domain action on such a large scale against even a few landowners could trigger “decades of court disputes before anything is built”, he told the Washington Post.

In the 2000s, the Bush administration had to negotiate land purchases with hundreds of landowners. Many citizens and local governments mounted resistance, causing major delays.

Among them was a family who lost half their farmland and their house and are now forced to live on the Mexican side of the fence, only accessing the US side via a locked gate into which they have to punch in a passcode.

Democratic Senator Claire McCaskill told the Senate Homeland Security Committee that out of 400 land acquisitions needed for the fencing that currently exists, 330 lawsuits were filed by the Department of Justice and more than 90 of those cases were still pending.

However, some landowners are receptive to the current administration’s moves.

The Villareal family, whose property lies just outside Rio Grande City, Texas, and borders the river, say they now feel better protected under President Trump because of his hardline approach to immigration.

“I do feel safer,“ says Daniel Villareal. “I mean, we've always come down here [to the river] armed, ever since we're sort of having the problem [of illegal immigrants crossing their land].

“But now that the situation's different, and there's not that many people trying to come across, maybe we can come down here, clear this area and then have our riverbank.”

The family was approached by the Bush administration when it was researching areas to build a fence, but nothing came of it. Rene Villareal, Daniel’s brother, says the family would not oppose plans for a wall across their property if it helped prevent more people crossing the border illegally in the short term.

“I want the wall because it's not going to be permanent,” says Rene. “It's not going to be forever into eternity. It's just until people come to their senses.”

But, while some private property-owners may not object, the proprietors of Tribal lands have already voiced firm opposition. The Tohono O’odham Nation owns much of such land, including a reservation that extends along 75 miles of the border in Arizona.

Tribe members still live on both sides of the border, considering the territory their ancestral lands, and have indicated they will attempt to block construction if the wall goes ahead.

Should that happen, Mr Trump would need a bill from Congress to acquire the land, which is currently protected under law.