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.