US Defence Secretary Jim Mattis meeting Defence Secretary Gavin Williamson (Picture: PA).
The US Defence Secretary Jim Mattis is resigning, a day after President Trump said he was pulling American troops out of Syria.
There's also growing speculation in the US media that the President is set to announce that half of the 14,000 US troops in Afghanistan are to be withdrawn.
It's thought President Trump overruled his Defence Secretary's advice on a Syrian pull-out and then pressed forward with talks on a partial Afghan withdrawal.
General Mattis will leave by the end of February after two years in the post. In his letter of resignation, he didn't spell out his exact reasons but wrote that he was leaving because:
"You have a right to have a secretary of defense whose views are better aligned with yours."
Mr Mattis also emphasised the importance of standing up for US allies writing:
"While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies."
The announcement came a day after President Trump announced the withdrawal of all 2,000 US troops from Syria. It's a move that has been sharply criticised for abandoning America's Kurdish allies, who may face a Turkish assault once they leave.
There's also speculation that General Mattis's decision to quit was prompted by Pentagon plans for a significant drawdown of 7,000 troops serving in Afghanistan, as reported by US broadcaster CNN and the Wall Street Journal.
Unnamed officials suggest the troops could be out by the summer but there's been no official confirmation of the plans.
It's no secret that Donald Trump has long pushed to pull troops out of Afghanistan.
Earlier this year, Defence Secretary Mattis and other military officials persuaded him to keep troops on the ground to pressure the Taliban and battle an Islamic State insurgency.
In light of the drawdown reports, Britain's Chief of the Defence Staff, General Sir Nicholas Carter, has said he believes the US remains committed to the international mission in Afghanistan.
General Carter said British lives had not been lost in vain in Afghanistan but he described the current security situation in the country as fragile.
In recent months there has been a renewed effort to make progress on peace talks with the Taliban.
But officials now worry that any move to withdraw US troops could dampen those prospects and simply encourage the Taliban to wait it out until they can take advantage of the gaps when the forces leave.