It was the centrepiece of the Trump election campaign. There was going to be a border wall, and at every rally he would shout out: "And who's going to pay for it?" And the crowd would roar back: "Mexico!" Except the Mexican government made clear there was no way it was going to fund it.
So when President Trump moved into the White House, he said that - initially - it might require US taxpayers to fund its construction, but the money would be clawed back from Mexico at a later stage.
Now with the president approaching his 100th day in office, he's come across another rock solid wall - the one presented by Democratic senators in opposition to his proposals.
They are able to block the spending proposals, and so the White House budget director will have to rewrite his plans - minus the funding for the border wall.
The president tweeted this morning: "Don't let the fake media tell you that I have changed my position on the WALL. It will get built and help stop drugs, human trafficking etc."
At some point in the future it may well get built, but this has been a stark lesson in the difference between campaigning and governing.
Mr Trump had proposed $1.5bn (£1.2bn) for his wall as part of the spending bill, which funds federal agencies to the end of the current fiscal year.
Analysts say the president is under pressure to deliver on his election pledges, few of which have been fulfilled during his first 100 days in office.
BBC News | Jon Sopel