By tweaking a course set by President Barack Obama, Trump suggests that he, like Obama and President George W. Bush before him, is facing the bleak reality of Afghanistan: There is no fast or politically palatable way to win, but losing quickly isn't an acceptable option, either.
"I think mush is what we have, and unfortunately I fear it's where we're headed," said Vikram Singh, a former senior adviser on South Asia at the Pentagon and the State Department who is now a vice-president at the liberal Centre for American Progress.
Military strategists, including Defence Secretary Jim Mattis, have long argued that there is no military solution in Afghanistan, and that the goal should be to convince the Taliban that they have more to gain from talking than fighting.
No military or diplomatic strategy has shown the homegrown insurgency that such a tipping point was at hand.
Trump set no time limit on how long the United States will remain.
"I've said many times how counterproductive it is for the United States to announce in advance the dates we intend to begin, or end, military operations," the President said.
In 2014, the military shifted from its combat mission in Afghanistan to two primary supporting roles: advising the Afghan security forces and going after terrorist groups in the country - al-Qaeda and later, Isis (Islamic State).
The Obama White House had wanted to end its time in office with a minimal number of forces in Afghanistan, mostly relegated to Kabul. Yet as US troops left and the Taliban began regaining territory, Obama decided against a total drawdown, and in June 2016 bolstered the advisory role and approved more aggressive airstrikes against the Taliban.
Trump will stick to that hedge, neither agreeing to pull out entirely nor attempting a major surge, as was seen in Iraq in 2007 and in Afghanistan in 2009.
"What are just a few thousand more troops going to do that hundreds of thousands couldn't do before?" asked Doug Ollivant, a senior national security studies fellow at the New America Foundation and a former Army officer. "It's the same forever war."
Afghan government forces and the militants remain locked in a stalemate, and the special inspector general for Afghanistan reconstruction recently assessed that the Taliban again control nearly half the country.
"We see this as a long-term campaign," said one US military official in Kabul. By sending more troops and investing in the war, he said, the United States and its Nato allies will be able to chart a way forward well into the 2020s when, the Pentagon hopes, the Afghan Air Force will be nearly self sufficient and Afghan President Ashraf Ghani's four-year road map toward government and military reform will be complete.
As a private citizen, Trump seemed to favour a more radical approach.
"We have wasted an enormous amount of blood and treasure in Afghanistan. Their government has zero appreciation. Let's get out!" he tweeted in November 2013.
Once in office, Trump put off any announcement of a strategy reboot, apparently frustrated that his advisers were not presenting him with options that could win the war.
The Pentagon has been pushing for more troops for months to add to the 8500 US forces there alongside 5000 soldiers from Nato countries. At the Pentagon, officials think the United States and Nato reduced its troops in Afghanistan "too far and too fast" after the end of combat operations in 2014.
In June, Trump gave Mattis the authority to add the troops he needed, after a request for more forces by General John Nicholson, the top US commander in Afghanistan. But Mattis held off, instead pushing Trump to own a long-term plan for the war.
Trump's announcement also zeroed in on Pakistan, long understood to harbour and support some of the militant groups, such as the Haqqani network and the Pakistani Taliban, that have trickled over the border and sown widespread violence in Afghanistan's provinces.
Pakistan has frequently denied harbouring militants and has accused Afghanistan of harbouring anti-Pakistani militants, even going so far as to propose a fence along its porous border. Last month, Mattis said that the United States would withhold US$50 million in defence funding from the country for not cracking down on Haqqani militants.
"We can no longer be silent about Pakistan's safe havens for terrorist organisations, the Taliban, and other groups that pose a threat to the region and beyond," Trump said. "Pakistan has much to gain from partnering with our effort in Afghanistan. It has much to lose by continuing to harbor terrorists."
For US forces in Afghanistan, the years of bloody battling - periods that included large combat operations and daily firefights - are in the past. US casualties are a fraction of what they were at the height of Obama's war - 11 this year as compared with nearly 500 in 2010.
"What we're seeing right now is the war we're fighting," said Marine Captain Brian Hubert, an officer who helps run a Marine command center in Helmand province, one of Afghanistan's most volatile regions.
US Special Operations forces run counterterrorism missions, and other American and Nato troops try to help the Afghans hold territory through training effective troops, intelligence and planning for offensive operations, and calling in airstrikes.
Longtime diplomat Jim Dobbins, a special representative for Afghanistan and Pakistan under Obama, called Trump's decision prudent.
"The choices come down to losing and not losing," he said.
"It would be nice to have an option that allows you to win in a 'War is Over' way, but that's been tried and found to be too expensive, was not sustainable and in the current circumstances, it's not politically feasible"
