Donald Trump has called on Nato to increase troops and funding for the war in Afghanistan "in line with our own".
In an address to the nation from Fort Myer, near Washington DC, on Monday night, the third US president to oversee what is America's longest war said sudden withdrawal would have "predictable and unacceptable" results.
He seemingly backed a Pentagon plan to shore up the Afghanistan government and end a stalemate with the Taliban by sending more forces after nearly 16 years of the war, but refused to discuss numbers.
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.
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 out own - we are confident they will."
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 defence secretary Jim Mattis.
Then president George W Bush started the involvement in Afghanistan when he ordered troops there after the 9/11 attack in 2001, 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.