Capt. Kristopher Farrar, an infantryman assigned to 1st Battalion, 1st Security Forces Assistance Brigade, advises his simulated partner during a Joint Readiness Training Center rotation Jan. 15 at Fort Polk, La. (Sgt. Arjenis Nunez/1st Security Force Assistance Brigade)

The silver bracelet Staff Sgt. Ryan Mathes wears memorializes the life of Sgt. Tanner Higgins, a friend and fellow Army Ranger who was killed in a firefight with the Taliban in 2012.

Mathes joined the Army in 2009 just days out of high school as the Obama administration surged the number of troops in Afghanistan to more than 100,000. He often faced combat in the following years with the elite 75th Ranger Regiment, as troop levels were eventually slashed and secure areas of the country again fell under Taliban control.

Now 27, he is preparing for his sixth deployment with a newly formed unit, as the Trump administration attempts to help the Afghans recover lost ground.

“For me, it’s a matter of not being fickle,” Mathes said of volunteering for his new assignment. “Everyone in our country can say easily that they believe in democracy. That they believe in feminism. Well, if you truly believe in those things, you should be convicted enough to go to war for them. Not just for yourself, but for others.”

As part of the Army’s newly created 1st Security Force Assistance Brigade, Mathes and about 800 other military advisers are expected to deploy this spring alongside a few hundred soldiers from the Army’s 3rd Infantry Division, who will provide security. The unit will be activated Thursday in a ceremony at Fort Benning, Ga.

The stakes are high. The SFAB could face more danger than any conventional U.S. unit in the war in several years because it is expected to advise and accompany Afghan kandaks, infantry units of a few hundred conventional soldiers, on a regular basis. U.S. troops advised such units during Obama’s surge, but only episodically in recent years.

The soldiers will fan out across the country in small teams, taking pressure off U.S. units that trained Afghan troops for years on an ad hoc basis, to varying degrees of success.

The deployment also will serve as a test case for the SFAB concept, which the Army hopes to expand to include six adviser brigades that could deploy around the world.

Col. Scott Jackson, the brigade’s commander, said the advisers’ deployment “is not a return to the days of old,” when U.S. troops faced firefights and explosive booby traps daily. The idea now is to enable the Afghan military to beat the Taliban, he said.

Still, he acknowledged that many of the soldiers in his unit have their own difficult memories of Afghanistan.

“They all have friends who made the ultimate investment and sacrifice in Afghanistan, and they want to see it through,” Jackson said. “Soldiers want good things. If they have a chance to make something better, they want to do it. There is no weariness in this formation or they wouldn’t be here if they were, right?”

Oct. 7, 2017 marks 16 years since the start of the U.S. war in Afghanistan. Here are key numbers to know about how that conflict has unfolded and how President Trump is changing it. (Sarah Parnass/The Washington Post)

Some lawmakers and Afghan war veterans are skeptical, in no small part because the unit’s first assignment will be in a long war that is close to the same age as its youngest soldiers. Despite years of U.S. troops deploying as advisers, the effort continues to sputter, according to a recent special inspector general report to Congress.

For the second consecutive year, the Afghan government’s control of its countryside has decreased, according to data released by the U.S.-led military headquarters in Kabul, the capital. The Afghan government had control or influence in 56 percent of its 407 districts as of October. Insurgents controlled 14 percent, with another 30 percent contested. In the fall of 2016, the government controlled 72 percent, with 7 percent under insurgent control and 21 percent contested.

Trump announced in August that he would not pull U.S. troops out of Afghanistan, despite his “first instinct” telling him to do so. The United States, he said, “must seek an honorable and enduring outcome worthy of the tremendous sacrifices that have been made, especially the sacrifices of lives.”

The new adviser brigade’s training schedule was shortened to deploy this spring as part of Trump’s new Afghanistan strategy, Politico and the New York Times reported recently. But the unit still had several months of training, including a school for advisers held at the unit’s home base of Fort Benning, Ga., and the 14 days of scenario-based training at Fort Polk, said Maj. Matt Fontaine, a brigade spokesman.

A battalion commander in the brigade, Lt. Col. Brian Ducote, said the training for this deployment is the best he has ever had, focusing not only on principles of warfare, but also on growing empathy for Afghans. Ducote, who has spent 6½ years of his life deployed to Iraq and Afghanistan, said that will be helpful, considering how dispersed the unit will be.

“What I’ve told the guys is that, ‘If you want to serve in this organization, if you volunteered for this organization, I am going to trust you at unprecedented levels that you would not necessarily typically have in another job doing something else,’ ” he said. “But with that trust comes a lot of responsibility, and you have to make sure that you can identify and assess and mitigate the risks without having someone with a lot more experience over your shoulder all the time.”

Defense Secretary Jim Mattis has said that concerns about the unit’s readiness for war have been “vastly overstated.” He plans to visit the brigade before it departs for Afghanistan, and has discussed the issue with senior military officials, he said.

“I have looked at the training regime and, first of all, the quality of these troops in terms of their experience and their selection,” Mattis said. “The training gives me a lot of confidence.”

Mattis has said repeatedly that the new U.S. strategy calls for forcing the Taliban to the negotiating table through strength. However, it’s unclear how much patience Trump has for that. The president, asked about recent suicide bombings in Kabul, said that “we don’t want to talk to the Taliban” and “we’re going to finish what we have to finish.”

Trump’s plan boosted the number of U.S. troops in Afghanistan from about 8,500 to 14,000, and up to 1,000 more could deploy. The Air Force has disclosed in recent days that a squadron of A-10C attack jets has deployed to Afghanistan’s Kandahar Airfield along with a few hundred airmen, boosting air support. The Air Force also has deployed additional MQ-9 Reaper drones and will soon send an additional combat search and rescue squadron, which rescue downed pilots.

At Fort Polk, the training of the Army advisers made clear that they are preparing for potential bloodshed. The soldiers not only planned military operations alongside role players pretending to be Afghan soldiers, but also accompanied them on complicated operations in villages filled with deadly surprises.

In one exercise, the role players — members of the 10th Mountain Division wearing Afghan flag patches on their uniforms — searched several buildings for a Taliban financier. Village elders allowed the soldiers in, but the financier hid from them, and a firefight erupted.

Ninety minutes later, U.S. and Afghan troops killed the last of several Taliban fighters — but only after five Afghan soldiers, one American soldier and one civilian were killed. Five additional U.S. soldiers and one Afghan soldier were wounded, said Maj. Ryan Swedlow, who helped oversee the training.

Capt. Wallace Rollins, a team leader in the brigade, said the exercises prepare U.S. troops for worst-case scenarios. Rollins, who served in Afghanistan in 2012 with the 82nd Airborne Division, indicated that some level of risk is necessary for the advisers.

“If we were to just plan with them and send them out on the mission, I don’t think we’d be able to build the strength of relationship and rapport needed to affect meaningful change,” he said.

Capt. James Gaffney, another team leader, recalled that on his 2013 deployment to Afghanistan as an adviser, some U.S. officers pushed their own agenda, seeking to show progress even if it meant requiring the Afghans to depend on American support for success. The advisers must now mentor Afghan forces and do everything they can to get them to win battles without help, and be ready to provide a “knockout punch” if necessary, he said.

“We want the SFAB to succeed,” Gaffney said. “We want a second SFAB, and we want a fifth SFAB. But if we don’t do our job right the first time, it will lose its momentum, and we don’t want to see that happen.”

Staff Sgt. Caleb Walters, who has deployed to Afghanistan twice before, said he joined the new brigade because he wanted to be part of history. He hopes that his unit can help put Afghanistan on a path where his 7-year-old son will never have to deploy there.

Walters said he suffered a “pretty terrible loss” in Afghanistan in 2013. He and another soldier attempted to rescue a friend, Sgt. Anthony R. Maddox, from a fuel fire that mortally burned him.

Asked to reflect on the risks that will go with this deployment, Walters paused for a second.

“If someone says they are not scared, I’d say they are a fool. Everyone should be scared to some capacity because I think that’s what keeps you on our toes and keeps you vigilant, if you will,” he said. “I’m not afraid to admit that to some capacity, yes, I’m scared. And there’s no reason for someone to say they’re not.”

Correction: An earlier version of this article misidentified the Army division that will provide security troops for U.S. advisers. This file has been updated