Our troops will fight to win," Trump said, and he insisted victory in Afghanistan would be clearly defined.
"Finally," the president said, "my administration will ensure that you, the brave defenders of the American people, will have the necessary tools and rules of engagement to make this strategy work."
"But we will no longer use American military might to construct democracies in far away lands, or try to rebuild other countries in our own image – those days are now over," Trump said. "Instead, we will work with allies and partners to protect our shared interests. We are not asking others to change their way of life, but to pursue common goals that allow our children to live better lives. This Principled Realism will guide our decisions moving forward."
Trump was widely expected to heed the advice of his senior advisors Monday evening and announce an increase of the number of US troops in Afghanistan, which currently stands around 8,400.
About 2,400 Americans have died in the war, the longest in the US’ history.
Trump began his remarks addressing the recent violent political divisions, most vividly seen in Charlottesville, Virginia, last week.
"When one part of America hurts, we all hurt," Trump said, adding, "Loyalty to our nation demands loyalty to one another."
Trump added that US troops fighting wars abroad "deserve to return to a country that is not at war with itself at home."
Trump acknowledged that his "instincts" told him to pull out troops from Afghanistan.
As a presidential candidate, Trump expressed frustration with former President Barack Obama’s policies in Afghanistan. Trump had tweeted to "get out" as recently as November 2013 and as early as August 2011.
"Historically, I like following my instincts," Trump said. He concluded differently, however.
"A hasty withdrawal," Trump said, would create a "vacuum" for terrorists.
Trump lamented the 20 terrorist groups operating in Afghanistan and Pakistan, the "highest concentration in any region, anywhere in the world," he said.
"Pakistan often gives safe haven to agents of chaos," he said, noting that there is nuclear-powered tensions between Pakistan and its neighbor India.
Trump called terrorists "nothing but thugs and criminals and, that's right, losers."
"We will not talk about numbers of troops," Trump said.
"Conditions on the ground," not public timetables, will determine the US' strategy, he said.
"I will not say when we are going to attack, but attack we will," he added.
"We are not nation building again. We are killing terrorists," he said.
Defense Secretary James Mattis released a statement saying that he had "directed the Chairman of the Joint Chiefs of Staff to make preparations to carry out the president’s strategy."
"I will be in consultation with the Secretary General of NATO and our allies—several of which have also committed to increasing their troop numbers. Together, we will assist the Afghan Security forces to destroy the terrorist hub," Mattis said.
However, Taliban spokesman Zabiullah Mujahid dismissed Trump's remarks on Afghanistan, saying they were "old" and "unclear," the Associated Press reported.
He went on to state that the Taliban will come out with a more detailed response in the future.
Last week the Taliban released a 1,600-word open letter to the president warning him against a troop surge. They also said that they are not ready for peace talks until the US and NATO give a timeline for troop withdrawal.