Trump claimed repeatedly during his campaign and into his term that Mexico would pay for a wall along the 2,000 mile border between Mexico and the United States.

Over the course of his term, Trump backtracked from his statements, claiming instead that it would be indirectly be paid for with Mexico tariffs, or through solar power generated by the wall, or other vague means.

Trump diverted billions of dollars from the Department of Defense to pay for the wall, which ultimately led to few new miles of wall and primary replaced preexisting fencing.

BBC News – Donald Trump: ‘Mexico will pay for the wall’
Politifact – Build a wall, and make Mexico pay for it 
The Washington Post – There’s new wall on 194 miles of the border. Sixteen miles didn’t have a barrier before.
Los Angeles Times – Trump campaigns on new border wall. There’s not much of it.
Politifact – “On the Southern border, as you know, the wall is going up, it’s going up very rapidly. We’re at 182 miles.” 
USA Today – A 2,000-mile search for answers