- The start of Trump's presidency marked a change in the transatlantic relationship.
- Trump implemented tariffs on European steel and aluminum, citing national security concerns.
- He has also been a vocal supporter of Brexit.
President Donald Trump is making a "historical mistake" when dealing with the European Union, a former U.S. ambassador told CNBC Tuesday.
"For 60 years, the U.S. administration, both Republic and Democratic, thought that European integration should be promoted because it was good for Europe and it was good for the U.S.," Anthony Gardner, former U.S. ambassador to the EU during the Obama administration, told CNBC's Joumanna Bercetche.
"This administration decided that European integration is bad and that the EU is in fact an enemy — that's the word that the president used, worse than a distraction, because it stands for things that this administration doesn't like."
The start of Trump's presidency marked a change in the transatlantic relationship, which included putting an end to talks over a wide-ranging trade deal. The Trump administration decided to implement tariffs on European steel and aluminum, citing national security concerns.
He said in 2018 that the EU is a "foe" of the United States and added that the 28-member bloc had been taken advantage of the U.S. in trade matters. The U.S. president is due to decide within the coming 24 hours whether to impose tariffs on European cars.
Trump has also called on EU countries to respect the defense targets established by NATO and has been a vocal supporter of Brexit.
Trump "prefers to deal one-on-one in a transnational way with particular states of the EU. I think this is a terrible mistake, I think it's actually a historic mistake and the reason I think that is because the EU is a natural partner of the United States on a lot of things," Gardner told CNBC at the UBS European Conference.
"Despite all the tensions, despite all the problems, we are natural partners and there's no hope for us to determine the rules of the road on trade, on digital economy and other things if we don't partner with our natural ally, the EU," Gardner said.
A spokesperson for the White House wasn't immediately available for comment when contacted by CNBC.