European officials knew the president’s win would threaten the fundamental precepts of the post-World War II order. But the speed at which it is unraveling has created a crisis of enormous proportions.

The US is likely to maintain its commitment to NATO, though it may reduce its troop presence in Europe as its priorities shift to the Indo-Pacific region, a top alliance official said.

The front-runner to be Germany's next chancellor, Friedrich Merz, said that Europe cannot rely on US President Donald Trump honouring NATO mutual defence obligations and must become more capable of defending itself.

A leading politician has voiced concern that the US administration might not uphold NATO's Article 5 - the cornerstone of the alliance.