President Donald Trump has said a hasty US withdrawal from Afghanistan would leave a vacuum for terrorists to fill.
He said his original instinct was to pull US forces out, but had instead decided to stay and “fight to win” to avoid the mistakes made in Iraq.
He said he wanted to shift from a time-based approach in Afghanistan to one based on conditions on the ground, adding he would not set deadlines.
However, the US president warned it was not a “blank cheque” for Afghanistan.
“America will work with the Afghan government, so long as we see commitment and progress,” he said.
The Taliban responded by saying that Afghanistan would become “another graveyard” for the US if it did not withdraw its troops.
Trump rhetoric versus Afghan reality
Taliban letter to Trump urges withdrawal
What is the new strategy?
President Trump has committed to stepping up the US military’s engagement in Afghanistan, but details were few and far between.
He said his new approach would be more pragmatic than idealistic, but he refused to get drawn on how many extra troops, if any, would be deployed and gave no timeline for ending the US presence in the country.
He did, however, put pressure on neighbouring Pakistan, warning that the US would no longer tolerate it offering “safe havens” to extremists – an accusation swiftly dismissed by a Pakistani army spokesman.
Can Afghan military turn the tide in Taliban fight?
Four days behind the Taliban front line
Tough school? War, illiteracy and hope in Afghanistan
The president also, for the first time, left the door open for an eventual peace deal with the Taliban, saying: “Someday, after an effective military effort, perhaps it will be possible to have a political settlement that includes elements of the Taliban in Afghanistan.”
However, Mr Trump said there would be an escalation in the battle against groups like al-Qaeda and so-called Islamic State.
“[They] need to know they have nowhere to hide – that no place is beyond the reach of American arms,” he said.
Meanwhile, Mr Trump made it clear he expects his existing allies – singling out India – to support him in his new strategy, and urged them to raise their countries’ contributions “in line with our own”.
Is Trump flip-flopping?
Before his presidency, Mr Trump was not shy about criticising his predecessors on their Afghanistan policy. He previously supported pulling US troops out of the conflict, which began under President George W Bush in 2001 after the 9/11 attacks.
Will it damage his base?
Aleem Maqbool, BBC North America correspondent, Washington
On paper, the Americans who could have the biggest problem with President Trump’s new Afghanistan strategy are the very ones who voted for him.
They were told Donald Trump would focus on a policy of “America First”, but he now says he wants a win in Afghanistan to make all the sacrifice worthwhile.
In terms of how he is going to achieve that goal, he did not lay out specifics – once again saying he does not like to signpost strategy to America’s enemies.
But it is hard to know what a modest troop increase would achieve that the massive surge under President Barack Obama could not.
It is also unclear how he expects more co-operation from Pakistan while also asking India to play more of a role in Afghanistan – the very thing that alarms the Pakistani establishment.
What is the reaction?
US Defence Secretary Jim Mattis indicated in a statement “several” US allies had already “committed to increasing their troop numbers”.
His UK counterpart, Sir Michael Fallon, said America’s commitment in Afghanistan was “very welcome”, adding: “We have to stay the course in Afghanistan to help build up its fragile democracy and reduce the terrorist threat to the West.”
General John Nicholson, the head of both US and international forces in Afghanistan, said the “new strategy means the Taliban cannot win militarily”.
But Taliban spokesman Zabiullah Mujahid dismissed Mr Trump’s strategy as “nothing new”, telling the AFP news agency that the US should think of an exit strategy “instead of continuing the war”.
US combat operations against the Taliban officially ended in 2014, more than 8,000 special forces continue to provide support to Afghan troops.
The Afghan government continues to battle insurgency groups and controls just half of the country.
bbc