Trump made the remark while standing beside Irish Prime Minister Micheál Martin in Dublin, telling reporters, “I’d love to see it unified… it would be a fantastic thing.” His words came just weeks after new British Prime Minister Andy Burnham declared that a referendum on Northern Ireland leaving the United Kingdom was “off the table,” a stance Downing Street confirmed had not changed.

The legal route to any such vote is set out in the 1998 Good Friday Agreement. The accord states that a referendum on Irish unity can be held only at the discretion of the British government.

The same agreement also ended three decades of violent conflict between Irish nationalists, who wanted a united Ireland, and unionists, who wanted Northern Ireland to stay in the U.K. By ending the bloodshed, the deal created a political framework for future decisions about the region’s constitutional status.

The leader of the Democratic Unionist Party slammed Trump’s prediction, saying unification would bring “the destruction of our country.”

Because the Good Friday Agreement gives the British government exclusive control over when a referendum can be called, any outside commentary – from Washington or Dublin – is seen as premature and politically sensitive. Leaders on both sides of the border therefore react strongly to such statements.