Kabul, Afghanistan (CNN) Secretary of Defense Ash Carter made an unannounced visit to Afghanistan Friday, a trip that comes amid major questions regarding the future of America's 15-year-long mission there.

Carter is set to consult with Afghan President Ashraf Ghani at the presidential palace in Kabul and meet with US military commanders and thank the troops at Bagram Air Base.

The Obama administration had originally intended to remove nearly all US troops from the country before leaving office. But faced with a lingering Taliban insurgency, the emergence of a local ISIS affiliate and the continued presence of al Qaeda terrorists, Obama revised the plan several times, eventually opting to hand-off the issue to his successor.

It is unclear, however, how the Trump administration will steer policy in Afghanistan, though his first appointments suggest that he will receive advice that favors a more robust American role in the fight there.

There are now 9,800 American service members in Afghanistan. The commander of the international coalition there, US Gen. John Nicholson, told reporters last week at the Pentagon that the number of US forces would be reduced to 8,450 by 2017.

There are also some 6,000 additional international troops participating in the NATO-led mission aimed at training and advising Afghan forces in their fight against terror groups. All told, it's a far cry from the 140,000 coalition troops that were fighting in Afghanistan five years ago.

While President-elect Donald Trump is due to inherit what some call America's longest war in a matter of weeks, he has said almost nothing about Afghanistan during the course of the presidential campaign and subsequent transition.

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"It's a mess. And at this point, you probably have to (stay) because that thing will collapse about two seconds after they leave," Trump told CNN's "New Day" in October 2015. Those comments were some of the only ones he has made about the US troop presence there.

But Trump will come into office being surrounded by significant expertise on the issue, as his choices to fill a number of key national security posts are rife with experience in Afghanistan.

Trump's long-time ally and selection for national security adviser, retired Lt. Gen. Michael Flynn, was hand-picked by Gen. Stanley McChrystal, the then-commander of the international coalition, to lead the international coalition's military intelligence efforts there in 2009 after Obama committed 30,000 US troops to the fight as part of an effort to stabilize that country.

In his 2016 book "Field of Fight," Flynn wrote that international terrorist groups remain a threat in Afghanistan.

"Al Qaeda hasn't been neutralized, in fact numerous al Qaeda leaders have relocated into that country," he said.

He also slammed the Obama's administration for drawing down troops from 2012 to 2014 after what he said were the counterinsurgency strategy's initial gains against the Taliban.

"Hardly anyone recognizes just how close we actually came to winning in Afghanistan," Flynn wrote. "But...winning isn't going to be durable if your next move is retreat."

And only weeks after 9/11, Carter's chosen successor, retired Gen. James Mattis, commanded some of the first US forces there, leading a brigade of US Marines. He was once again posted to Afghanistan in 2006.

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Mattis had some controversial words for the Taliban, famously telling a panel in 2005, "You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil. You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them. These people are fun to kill."

Mattis also oversaw the war in Afghanistan as part of his responsibilities while commander of US Central Command from 2010 to 2013. In 2006, along with then-Gen. David Petraeus, who is also rumored to be under consideration for a key post, Mattis wrote the military's counterinsurgency manual.

In it, they said that key to defeating insurgencies like the Taliban is to pursue a multi-pronged approach, including robust military, economic and political measures. If Mattis were to recommend such an approach to Trump, it's possible the US might reinforce its presence in Afghanistan with additional troops and resources.

And Trump's pick to run the Department of Homeland Security, retired Gen. John Kelly, who lost a son fighting in Afghanistan, made similar prescriptions in January when asked what was needed to be done about Afghanistan.

"I don't believe that we can allow Islamic extremists to ... have safe haven," he said, adding, "You have to do social, economic, military action, political action to prevent that."

He told Pentagon reporters at the time, "Some of the recommendations might be distasteful or out of the box in terms of some of the policy makers thinking."

"This is really hard, and we know how to do it," he added.

Nicholson, the commander of US forces, has admitted that the current troop levels in Afghanistan leave a "moderate level of risk" for the mission, noting that trainers and advisers will continue to be required to provide air support and improve leadership skills within the Afghan National Army for some time to come.

Obama only in June permitted the US military to carryout airstrikes against the Taliban. Previously, US forces were solely allowed to target ISIS, al Qaeda and any other forces directly threatening US personnel.

The Afghan air force is just now beginning to conduct its first independent airstrikes, and the government in Kabul has become increasingly reliant on Afghan Special Forces to carry out the fight against ISIS and the Taliban.

That relatively small 17,000-member Afghan force, which Nicholson called "arguably the best in the region," is responsible for 70% of offensive military operations, an operational tempo that Nicholson acknowledged is difficult to sustain.

Nicholson said that the members of the international community had pledged millions of dollars and advisory support to the Afghans until at least 2020, adding that these commitments would help grow the size of the Afghan Special Forces.

Even with the US providing advisers and airstrikes to the Afghans, the US military believes that the government only controls about 64% of the country, with the Taliban controlling about 10% and the remainder being contested by the army and the insurgency.

Nicholson acknowledged these challenges last week, saying the insurgency was "very difficult for the Afghans to defeat," but noted that that the Afghan army had successfully prevented the Taliban from seizing any major population centers in 2016.

US Special Operations Forces in Afghanistan are also carrying out counterterrorism operations targeting ISIS and al Qaeda fighters, often operating in partnership with elite Afghan troops.

Afghanistan and Pakistan represent "the highest concentration of terrorist groups anywhere in the world," Nicholson said, saying that US-led operations in 2016 had killed or captured 50 leaders from al Qaeda and its Indian affiliate, al Qaeda in the Indian Subcontinent.

But these type of counterterrorism missions can cause US troops to find themselves in harm's way, and several have been killed on missions in the last few months.

The US and its 39 coalition partners in Afghanistan committed to providing support to Afghanistan for through 2020, Given Flynn and Mattis's familiarity with the war there, it is likely they are already formulating recommendations as to what to do next about America's involvement.

"It's been a long fight, 15 years. But I think it's one we got to remember this--their actions, their sacrifice, are protecting our homeland," Nicholson said of the military's effort there.

"Counterterrorism and training, advising and assisting the Afghan security forces: It's a very, very sound policy," Nicholson said. "This is the course that we're on, and I know that's what this administration will hand off to the next administration."