Addressing Ireland’s parliament in Dublin on Wednesday, U.S. House Speaker Nancy Pelosi seized control of American foreign policy in relation to Brexit, saying that Congress would block any new trade deal with the United Kingdom if Britain’s exit from the European Union threatens the peace in Northern Ireland.

After introducing the slew of Irish-American lawmakers traveling with her, Pelosi praised the Good Friday Agreement, which brought peace to the British-governed territory of Northern Ireland in 1998 and removed the need for customs and security checkpoints along its border with Ireland.

“America will continue to stand with you in protecting the peace that the Good Friday accords have realized,” Pelosi said. “I’ve said it before and I’ll say it again: We must ensure that nothing happens in the Brexit discussions that imperils the Good Friday accord, including, but not limited to, the seamless border between the Irish Republic and Northern Ireland.”

“Let me be clear,” Pelosi added, “if the Brexit deal undermines the Good Friday accords, there would be no chance of a U.S.-U.K. trade agreement.”

Pelosi also recalled “the bravery of our late friend, the extraordinary Martin McGuinness.” Before renouncing violence for politics, McGuinness was a commander in the Irish Republican Army. “Martin is beloved and missed by his many friends in Congress,” Pelosi added.

Pelosi’s words on trade were a hammer blow to so-called Brexiteers in Britain, who have fed the hearts of their supporters on the fantasy that the U.K. can simply walk away from the European customs union and single market, and offset the damage to its economy by striking a free trade deal with the U.S.

The stumbling block to that plan is that the U.K. can only trade freely with the U.S. after it completely withdraws from the EU, but doing so would require customs and immigration checks between Ireland, which remains an enthusiastic member of the European bloc, and Northern Ireland, which would leave.

Perhaps the greatest achievement of the peace in Northern Ireland has been the decrease in tension along that now-invisible line of partition, imposed on Ireland in 1921, when the British Empire divided the country in two. The prospect of once again fortifying it is seen by observers on all sides as a risk to the entire peace process.

To preserve the peace, EU negotiators initially proposed making Northern Ireland a special economic zone, which would remain inside Europe’s economic bloc with the rest of Ireland after Brexit. That proposal was popular with Northern Ireland’s business community — and the majority of the region’s voters, who opposed Brexit in the 2016 referendum — but was ultimately rejected by the British government.

Prime Minister Theresa May, who depends on the support of a small party from Northern Ireland committed above all to maintaining the union with Great Britain, proposed instead that the whole of the U.K. would remain in a customs union with the EU after Brexit if no other solution could be found to keeping the border open in Ireland. The catch, for those dreaming of a post-Brexit trade deal with the U.S., is that countries that are in the European customs union are barred from striking their own independent trade deals with other nations.

That has led the most hardcore Brexit supporters in the British Parliament to propose simply leaving the EU without any special arrangement for Northern Ireland and taking U.S. President Donald Trump up on his offer of a comprehensive trade deal with the U.S.

Trade deals, however, have to be approved by Congress, which has, for decades, been packed with influential Irish Americans of both parties. Among those traveling with Pelosi to London and Dublin this week is Rep. Richard Neal, a Massachusetts Democrat who helped broker the Good Friday Agreement and is the current chair of the House Ways and Means committee, which oversees trade policy.

Neal is also the co-chair of the Congressional Friends of Ireland caucus with Rep. Peter King, a New York Republican. Another member of that caucus accompanying Pelosi, Rep. Brendan Boyle, told the Irish Times that the delegation had a heated exchange with members of the pro-Brexit European Research Group, led by Conservative Member of Parliament Jacob Rees-Mogg, in London on Monday.

Boyle, whose father was born in Ireland, said that the delegation attempted to provide “a reality check” to those in Rees-Mogg’s group who claimed “that the border issue is ‘concocted'” by a secret cabal of anti-Brexit politicians “in London, Brussels, Dublin, and Washington, all in some sort of grand conspiracy to force them to do something that they don’t want to do.”

“We very much attempted to disabuse them of that sort of conspiracy-type thinking,” Boyle said.

Rees-Mogg’s group continues to insist that any border checks after Brexit could simply be handled by technology. The fact that no such technology appears to exist anywhere in the world has not deterred them.

The fanciful nature of that supposed solution was underlined on Tuesday by Karen Wheeler, the British customs agency’s senior official in charge of post-Brexit border planning. “There is no technology solution which would mean that you could do customs controls and processes and not have a hard border,” Wheeler told business leaders meeting on Tuesday night at a Belfast museum dedicated to the Titanic, which was built in the city. “There is no magic solution that would make that go away. If there was, trust me, we would have found it.”

As the Washington Post reported, Pelosi also confirmed that, during her delegation’s earlier stop in London, she delivered exactly the same message to both the British prime minister and opposition leader Jeremy Corbyn. “We made it clear to all that if there were any harm to Good Friday accords — no treaty,” Pelosi said. “Don’t even think about that.”