John Kerry, a key figure in the Biden administration as US Special Presidential Envoy for Climate, was serving as Secretary of State in 2016 under former US President Barack Obama.
Mr Kerry appeared alarmed after German Chancellor Angela Merkel, then-French President Francois Hollande and then-Italian Prime Minister Matteo Renzi vowed to get tough on Britain in the immediate aftermath of the Brexit referendum.
Holding an emergency summit, the three leaders insisted there would be no discussions at all with the UK about its breakaway until Article 50 was triggered.
Mr Hollande went one step further, urging Britain to “not waste time”.
Although comforting in Brussels, this united front did not go down well on the other side of the Atlantic, as Mr Kerry called for calm and argued against “revengeful promises”.
He said: “I think it is absolutely essential that we stay focused in this transitional period.
“Nobody loses their head, nobody goes off half-cocked, people don’t start ginning up scatterbrained or revengeful premises.”
Mr Kerry’s comments came as former Prime Minister David Cameron travelled to Brussels for his last EU summit.
“To punish someone suggests emotion, anger or a desire for revenge but, when it comes to Brexit, the Europeans have said they are surprised at how emotional the UK has been.
“After all, the British have this reputation for a stiff upper lip and keeping calm and carrying on.
“It dates back to World War 2.”
Ms Adler then claimed that the EU blame this revenge narrative on internal squabbles within the UK, rather than a Brussels-based agenda.
She continued in 2018: “The EU says that the UK Government has spent so much time arguing and fighting with itself, that that has slowed down negotiations.
“Brexit negotiations are also hard-going because of pure self-interest.
“Think about it. In any deal, whether it’s business or political, each side naturally wants to come out on top.
“So the EU would say, it’s not punishing the UK, it’s just looking after itself.”