US President Donald Trump says he is replacing Defence Secretary Jim Mattis two months earlier than expected, a move officials say was driven by Mr Trump's anger at Mr Mattis' resignation letter and its rebuke of his foreign policy.
On Thursday, Mr Mattis had abruptly said he was quitting, on February 28, after falling out with Mr Trump over his foreign policy, including surprise decisions to withdraw all troops from Syria and start planning a drawdown in Afghanistan.
Mr Trump has come under withering criticism from fellow Republicans and international allies in recent days over his moves to wind down US involvement in Syria and Afghanistan, against the advice of his top aides and US commanders.
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The exit of Mr Mattis, highly regarded by Republicans and Democrats alike, added to their concern over what they see as Mr Trump's unpredictable, go-it-alone approach to global security.
In announcing his resignation, Mr Mattis distributed a candid resignation letter addressed to Mr Trump that laid bare the growing divide between them, and implicitly criticised Mr Trump for failing to value America's closest allies, who fought alongside the United States in both conflicts. Mr Mattis said that Mr Trump deserved to have a defence secretary more aligned with his views.
Mr Trump made his displeasure with Mr Mattis clear on Saturday night by tweeting that he had been "ingloriously fired" by former President Barack Obama and he had given Mattis a second chance. Mr Obama removed Mattis as head of US Central Command in 2013.
On Sunday, Mr Trump tweeted that Deputy Defence Secretary Patrick Shanahan would take over for Mattis on an acting basis on January 1. In a tweet, Mr Trump called the former Boeing Co executive "very talented".
In his letter, Mr Mattis had said he would step down at the end of February to allow for a successor to be confirmed and attend Congressional hearings and a key NATO meeting.
A senior White House official said that Mr Trump was irked by the attention given to Mattis' resignation letter.
"He just wants a smooth, more quick transition and felt that dragging it out for a couple of months is not good," the official said, on condition of anonymity.
The official said Mr Trump was expected to pick a nominee for defence secretary over the next couple of weeks.