Jim Mattis has resigned as US defence secretary to protest the way Donald Trump has treated allies and dealt with adversaries such as China and Russia, in a dramatic exit that came a day after the president said he would withdraw forces from Syria.

Mr Trump tweeted on Thursday that Mr Mattis would be “retiring”. But in a resignation letter, Mr Mattis made clear he was leaving because he disagreed with the commander-in-chief over a wide range of policies and suggested that Mr Trump was putting the nation at risk. Mr Mattis opposed withdrawing from Syria.

“Because you have the right to have a secretary of defence whose views are better aligned with yours?.?.?.?I believe it is right for me to step down from my position,” Mr Mattis wrote in the letter, which was released shortly after a meeting with Mr Trump.