Why Trump's united Ireland stance hit such a nerve

President Trump stood by his support for a united Ireland on Sunday after his remarks during a two-day visit sparked a backlash from unionist and British politicians and praise from Irish nationalists.

Why it matters: Trump broke with a longstanding U.S. presidential convention of not taking sides on whether Northern Ireland should ultimately be British or Irish.

• The U.S. helped broker the 1998 Good Friday Agreement, built around consent and parity between British unionism and Irish nationalism.
• Former President Clinton, whose administration played a central role in the peace process, explicitly said the U.S. was "neither in favor of a united Ireland nor opposed to the idea."

Driving the news: Trump said in Dublin, sitting beside Irish Taoiseach Micheál Martin on Saturday, that he'd "love to see" Ireland unified and that it is "going to happen eventually," so it "may as well happen now."

• He described uniting Northern Ireland with the Republic of Ireland as "fantastic," adding: "The U.K. would have something to say about it obviously, but I think it would be a great thing, I mean, how can it be bad?"
• Trump later said Irish reunification would be a "very cool thing" while noting that "no matter what you say or how you say it, there's no good answer."

On Sunday, Trump told reporters: "It just seems to me that you have Northern Ireland and you have Ireland. It seems to me that one of the naturals of all time is to put them together."

• He added this was only his opinion, but "many people agree" and he "had so much support" in response to his remarks.

Between the lines: Trump has managed the unusual feat of delighting Irish nationalists while alarming some Northern Irish politicians who have previously expressed support for him.

• Martin revealed that Trump has privately expressed the same preference before.

What they're saying: While Irish nationalist politicians seized on Trump's comments and said "the conversation on Irish unity has shifted," unionists in Northern Ireland and politicians in Westminster swiftly condemned the remarks.

• Democratic Unionist Party leader Gavin Robinson, who represents East Belfast in Parliament, said on X, "Northern Ireland's constitutional future will be decided by the people of Northern Ireland, not by commentary in London, Dublin or Washington. Northern Ireland is part of the United Kingdom because that is the settled democratic will of its people."
• Robert Jenrick from Nigel Farage's Reform Party said on X that Trump was "completely wrong," adding: "Northern Ireland is, and will always be, a proud part of the United Kingdom."

A 10 Downing Street spokesperson said in a media statement that British Prime Minister Andy Burnham's position on Irish unification remains unchanged from previous comments he's made that a referendum on a united Ireland was "off the table" as he's "not aware ​that there is majority public support" for one.

• The spokesperson added that Burnham is committed to upholding the Good Friday Agreement, which largely ended three decades of conflict known as the Troubles, established Northern Ireland's current system of government and largely removed visible signs of the border.

Context: Under the Good Friday Agreement, Northern Ireland remains part of the U.K. unless voters in Northern Ireland choose reunification and voters in Ireland approve the constitutional change — putting democratic consent, rather than the preference of London, Dublin or Washington, at the center of its constitutional future.

Flashback: It's not the first time that Trump has floated the idea of a united Ireland.

• In March, he joked about a Northern Ireland-Republic "merger" in Washington.