Donald Trump has said he is confident that Nato allies such as Britain will increase troops and funding for the war in Afghanistan "in line with our own".
But the third US president to oversee what is America's longest war remained secretive over what the increases will be during an address to the nation from Fort Myer, near Washington DC, on Monday night.
Mr Trump, who made repeated calls for the nearly 16-year war to end ahead of his election, said sudden withdrawal would have "predictable and unacceptable" results.
His announcement appeared to support a Pentagon plan to shore up the Afghanistan government and end a stalemate with the Taliban by sending more forces.
In a speech evoking the 9/11 attack in 2001 which drew the US into the war, as well as the recent terror in Barcelona, Mr Trump said he believed allies would back increases.
"From now on victory will have a clear definition: attacking our enemies, obliterating Isis, crushing Al Qaeda, preventing the Taliban from taking over Afghanistan and stopping mass terror attacks against America before they emerge," he said.
"We will ask our Nato allies and global partners to support our new strategy with additional troop and funding increases in line with our own – we are confident they will."
US defence secretary Jim Mattis said in a statement that he would consult with Nato and other allies, adding that "several" had already committed to increasing troop numbers.
Before his election, Mr Trump made repeated calls for the US to withdraw, saying lives and money were being "wasted".
But Monday's announcement came after discussions with top Pentagon advisers and the intelligence community as well as vice president Mike Pence and Mr Mattis.
Mr Trump said he would not put a timescale on the end of the war but would instead wait for certain conditions to be met on the ground.
He declined to discuss further details on troop numbers, saying: "America's enemies must never know our plans."
He also addressed his reversal in opinion which he said comes after studying Afghanistan from "every conceivable angle".
"My original instinct was to pull out and historically I like following my instincts, but all my life I've heard that decisions are much different when you sit behind the desk in the Oval Office," Mr Trump said.
He also issued a warning to Pakistan, urging it to stop providing "safe havens" for terrorist groups.
"Pakistan has much to gain from partnering with our effort in Afghanistan, it has much to lose by continuing to harbour criminals and terrorists," he said.
Then-president George W Bush sent troops into Afghanistan after the 9/11 attack and while his successor, Barack Obama, increased the military presence to over 100,000 and failed to bring it to a close as he had planned.
Currently there are around 8,400 US troops in the country and Pentagon officials had proposed sending in a further 4,000 to train Afghan forces and fight the Taliban and an affiliate of terror group Islamic State.
There are also about 500 British troops there and around a further 85 had been promised in non-combat roles.