Ireland's EU Commissioner Michael McGrath has said the transatlantic relationship is at a "pivotal and dangerous" moment.
He warned that if the Trump administration pressed ahead with tariffs on those countries who have opposed the US President's ambitions to seize Greenland, there would be a "robust" response from the European Union.
Speaking to RTÉ News in Davos, Mr McGrath said: "We are at a moment of danger in the overall transatlantic relationship.
"It has been the bedrock of peace and security and stability for the world over many decades, and everybody should think long and hard about damaging that relationship, because if that breaks down, and if it is a case that the strongest militarily and politically and economically will always get their way, and can set aside international law, set aside the rules based system, set aside the role of the United Nations, then we're in a very dark place," he said.
"But in the event that this all goes wrong, and we see unilateral action from the United States, then, of course, we will respond, and it will be a robust response."
The commissioner said the EU and US had negotiated a trade settlement following many months last year.
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"We expect the terms of that agreement to be fully honoured by the US, as they would expect us to do the same, and that provides the stability and certainty and predictability that businesses crave at a time when there's so much uncertainty in the world.
"To come now and seek to reopen and revisit a hard won negotiation that resulted in an agreement with compromises and remade sacrifices on the EU side - for sure, in the broader interest of achieving an agreement - it's not possible now to have a fundamental reopening so soon."
Mr McGrath said the crisis over Greenland meant the EU-Mercosur trade deal must now be seen in a new light.
"What all of this uncertainty underlines is the need for Europe to diversify its trading relationships and to build new alliances, new relationships and to open up new markets for European companies.
"This is what we're good at. Europe is exceptionally strong at international trade, exporters of goods and services seeking out opportunities all over the world.
"Mercosur opens up a market of over 700 million people, [with] the elimination of tariffs on over 90% of product lines. It's a massive opportunity for Europe and for Ireland.
"We have listened very carefully to all of the legitimate concerns that have been expressed by the farming community, and I think we have put in place robust defence mechanisms to make sure that those fears don't come to pass," he said.
