The Trump administration is planning to withdraw thousands of troops from Afghanistan, US media say.
Reports, citing unnamed officials, say about 7,000 troops – roughly half the remaining US military presence in the country – could go home within months.
The reports come a day after the president announced the country’s military withdrawal from Syria.
In his letter of departure, Gen Mattis mentioned his diverging views with the president, but did not cite troops withdrawals directly.
Before his election, Mr Trump repeatedly publicly advocated leaving Afghanistan, but last year he indicated he would keep boots on the ground indefinitely to prevent the country’s collapse amid a Taliban resurgence.
Reports about the sharp reduction of forces emerged on Thursday, but have not been confirmed by US defence officials.
What is the story of the US in Afghanistan?
The US has been in Afghanistan since 2001, after the 11 September attacks – the longest war in US history.
When the Taliban, who controlled Afghanistan, refused to hand over al-Qaeda leader Osama bin Laden – who had claimed responsibility for the attack- President George W Bush launched a military operation to find Bin Laden and remove the Taliban from power.
US special forces eventually found and killed Bin Laden in Pakistan in 2011 and American-led combat operations against the group in Afghanistan officially ended in 2014.
But in the years since then, the Taliban’s power and reach have soared – and US troops have stayed on the ground in an effort to stabilise the country.
In September 2017 Mr Trump announced the US would send 3,000 extra troops to the country, which was a clear shift from his previous rhetoric.
What has reaction been?
The Washington Post reports the potential move is being met with opposition by some of Mr Trump’s senior cabinet officials including outgoing White House Chief of Staff John Kelly and White House national security adviser John Bolton.
Republican senator Lindsey Graham tweeted that any withdrawal of troops would be a “high risk strategy” which could reverse US progress in the region and pave the way toward a “second 9/11”.
He had earlier spoken out against the Syria withdrawal, which he described as a “huge Obama-like mistake”.
Some groups warned the move could create a vacuum which would allow groups like the depleted Islamic State (IS) to revive.