What did Donald Trump do today?

He forgot that most countries don't want border walls.

Trump appeared at a joint press conference with Irish Prime Minister Leo Varadkar in a conference room at the Shannon airport. Normally, this kind of meeting would have taken place at the ministerial estate in Dublin, but Trump's actual reason for coming to Ireland today was to visit his golf course, and Shannon was the closest airport.

Trump had barely begun speaking when he volunteered this:

TRUMP: We’ll be discussing various things. Probably, you’ll ask me about Brexit because I just left some very good people that are very much involved with Brexit, as you know. And I think that’ll all work out. It will all work out very well. And also, for you, with your wall, your border. I mean, we have a border situation in the United States, and you have one over here. But I hear it’s going to work out very well. I think it’s both going to work out well. It’s going to work out very well here.

This forced Varadkar to interrupt with a gentle reminder:

PRIME MINISTER VARADKAR: The main thing we want to avoid, of course, is a border or a wall between both sides.

Trump appeared confused in his response to that, contradicting himself almost immediately:

TRUMP: No, I think you do. I think you do. The way it works now is good. You want to try and keep it that way. And I know that’s a big point of contention with respect to Brexit, is your border.

Trump is wrong about what that "point of contention" is. Ireland and the country of Northern Ireland (which is part of the United Kingdom) were caught up in a violent conflict for much of the late twentieth century, over the question of whether Northern Ireland should be unified with Ireland. Several thousand people were killed. "The Troubles" only came to an end in 1998.

The normalization of the once-militarized border between Ireland and Northern Ireland, which in many places passes through individuals' backyards and farm fields, is a symbol of the progress that has been made in defusing that conflict. (The "walls" that were built during the Troubles are now tourist attractions, and in many cases are being dismantled.)

But having an open border between a post-withdrawal UK and Ireland, an EU member, would make it all but impossible to regulate the flow of goods between the EU and the UK. This is the "big point of contention," although one of the few things that all parties involved agree on is that no one wants the return of a hardened border.

Pressed for further comment by a reporter, a visibly shaken Trump retreated from specifics:

TRUMP: No, I think it should be good. I mean, the big thing is going to be your border, and — hopefully that’s going to work out. I think it will work out. There are a lot of good minds thinking about how to do it, and it’s going to be — it’s going to be just fine. I think, ultimately, it could even be very, very good for Ireland. But the border will work out.

Trump, who sometimes refers to himself as "Mr. Brexit," did not volunteer any further opinion about what those "good minds thinking about" the border issue should do.

Varadkar later told reporters that he had explained the situation to Trump in the meeting they had after the conference.

So what?

• It's bad to try to speak about things you know nothing about.

• Insulting America's allies, intentionally or just because you want to play golf, does actual harm to America.