Joe Biden made a fundamental error in a speech at NATO's summit in Madrid earlier today, referring to Switzerland joining NATO instead of Sweden.
Praising the two new additions to the defence alliance – Finland and Sweden – the 79-year-old slipped up, incorrectly referring to Switzerland.
Rectifying his error, Mr Biden joked: "My goodness, I'm getting really anxious here about expanding Nato".
He described the two nations' membership to the alliance as a "historic act" in retaliation to mounting aggression from Moscow towards European member states.
Mr Biden said: "We've invited two new members to join NATO. It was a historic act."
Continuing his statement, he added: "We got on the telephone, he suggested we call the leader of, Switzerland. Switzerland! My goodness. I'm getting really anxious here about expanding NATO, of Sweden!"
U.S. President Joe Biden gestures as he boards Air Force One to depart for Washington from Madrid
JONATHAN ERNST
Switzerland has refrained from joining NATO, in compliance with its longstanding policy of neutrality.
The Republican National Committee condemned the President for his blunder, calling him out on the rookie error.
Mr Biden has made a number of mistakes on the world stage including one about Taiwan.
The prevalence of mistakes in speeches made by Mr Biden have prompted increased questioning of the mental state of the American President.
Concerns mounted about his age and low approval ratings, following record-high inflation and spiking gas prices.
Mr Biden's attendance at the three-day NATO summit in Madrid was designed to cement the unity of the alliance, which had been marred under his predecessor, Donald Trump.
Speaking after the event, Mr Biden said: "I think we can all agree that this has been a historic NATO Summit.
"This summit was about strengthening our alliances, meeting the challenges of our world as it is a day, and the threats we're going to face in the future."
The NATO summit reached a remarkable landmark earlier this week, when Turkey revoked its decision to exclude Finland and Sweden from joining.
Mr Biden told reporters on Thursday: “Putin wanted the 'Finland-isation' of NATO. He got the NATO-isation of Finland."
The American President slipped up during a speech at the NATO summit in Madrid
JONATHAN ERNST