Britain was not told of American plans to withdraw up to half its troops from Afghanistan, with officials only hearing of the abrupt shift by its closest military ally from news reports.
The decision caused surprise and unease among Western diplomats in Kabul, with one warning it was an early Christmas present to the Taliban.
Only hours before reports of the plan, the head of the British armed forces had said he believed Donald Trump would not announce a major troop withdrawal, as he had from Syria earlier this week.
Gen Sir Nick Carter, Chief of the Defence Staff, told the BBC he believed America was “wholeheartedly committed” to the international coalition propping up the Afghan government against a Taliban insurgency.
Mr Trump has reportedly ordered generals to begin withdrawing up to 7,000 of the 14,000 US troops currently helping the Afghans battle the Taliban and Islamic State group.
“ It was a surprise, we weren’t expecting it. Now we need to see exactly what the detail is, before we see what we are doing.” said one British defence source.
The Afghan presidency tried to brush off the announcement, saying its forces were largely in control of security rather than Americans.
But the disclosure was a gift to the Taliban, who earlier this week met US envoys for tentative peace discussions in Abu Dhabi, said one Western official.
"If you’re the Taliban, Christmas has come early," a senior foreign official in the Afghan capital told AFP.
"Would you be thinking of a ceasefire if your main opponent has just withdrawn half their troops?"
The withdrawal of American troops has been the key demand of Taliban envoys, but America has sought a ceasefire, assurances terrorists will not be able to use Afghan territory and discussed having permanent bases.
It was unclear if the withdrawal plan was related to this week’s three-day talks with US envoy Zalmay Khalilzad, but it appeared to come with no strings.
Mr Trump announced on Wednesday he was pulling out all 2,000-odd US troops fighting in Syria because he claimed they had defeated Islamic State in Iraq and Levant (Isil).
Sir Nick, was asked during a visit to Kabul if he was worried Mr Trump would do the same in Afghanistan, where Britain this year nearly doubled its troop presence to 1,000.
He said: "I think the commitment I have seen over the last 24 hours would suggest that this is not happening here. It’s a different mission [to Syria]."
Mr Trump has long questioned why America is engaged in Afghanistan. America’s longest ever war has seen the country pour vast sums of money into the conflict and still see the Afghan government struggle to control or influence half the country.
The US president was last year persuaded by military advisers to pull out only if security got better and in the meantime increased air strikes and troop numbers in a new strategy he vowed would see troops “fight to win”. But the Pentagon has this year admitted the conflict remains a stalemate, while Afghan forces and civilians have seen sharply growing casualties.
Diplomats in Kabul have long talked of a ‘Tweet of Damocles’ hanging over them from Mr Trump, in which he would decide abruptly to change his mind and announced on social media he was pulling out.
"The US has not consulted us on the withdrawal and today we will start meetings to discuss it," a Western diplomat in Kabul told Reuters.
"It will take a while and there are some countries who are ready to exit. So they could be the first to leave."
Mr Trump’s decision not to inform the UK in advance was the latest blow to the "special relationship" between the two countries under Mr Trump’s administration.
The US president has also previously rebuked Theresa May for her handling of Brexit, including in a bad-tempered call from Air Force One.
Last month he called her approach to Brexit a "great deal for the EU" that could make it difficult for the UK to trade with the US.