U.S. Marines of the 1st Battalion, 6th Marine Regiment build a barrier around their base in the southern Afghan town of Marjah. (Dusan Vranic/AP)

President Trump’s speech this week announcing his decision to extend the U.S. war in Afghanistan provided no detail on exactly what new American troops will do when deployed.

Trump went out of his way during his announcement at Fort Myer, Va., to say, “we will not talk about numbers of troops or our plans for further military activities.”

Trump’s decision not to telegraph his plans was in keeping with his frequent assertion on the campaign trail about the need to maintain battle plan secrecy — a stance that was intended as a rebuke of former president Barack Obama’s 2009 announcement in which he provided a timetable for withdrawal from Afghanistan.

So what is in store for U.S. troops in Afghanistan and how many forces might Trump send?

The top U.S. commander in Afghanistan, Army Gen. John Nicholson and his boss, Gen. Joseph Votel, head of U.S. Central Command, testified to Congress earlier this year that they needed “a few thousand” more troops to help push back the Taliban and support the Afghan military. Eventually, the Pentagon settled on 3,900, a number repeated up until Trump’s announcement.

It is unclear how many U.S. troops are in Afghanistan. Officially, there are supposed to be around 8,500, but with Special Operations forces and other units constantly rotating in and out of the country, that number is likely somewhere around 10,000 to 11,000. There are also about 5,000 troops from other NATO and coalition countries.

More troops will reinforce the two existing missions — Operation Resolute Support and Operation Freedom Sentinel. The former is a joint U.S.-NATO operation to train, advise and assist Afghan security forces, including the army, air force and special operations units. The latter is a U.S.-run counterterrorism mission focused primarily on al-Qaeda and the Islamic State.

“What we’ve asked for is primarily forces for the train and advise and assist portion,” said a U.S. military official in Kabul, speaking a week before Trump’s announcement. “It’s is a very small portion that has a counterterrorism flavor to it.”

“More than 95 to 98 percent of what we’ve asked for is the [train, advise, assist] mission,” said the official, who spoke on the condition of anonymity to discuss planning.

The extra troops, the official said, would give U.S. forces the resources to advise below the higher echelons of Afghan Army corps. In 2011, when there were more than 100,000 U.S. troops in Afghanistan, Americans were advising at the squad level — meaning that for every 12 Afghan troops, there were 12 American troops alongside them.

By adding, say, 3,900 troops to the war effort, U.S. forces will be able to move down toward brigade (4,000 troops) and battalion (670 troops) levels.

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Advising at smaller units means U.S. troops will get closer to the fighting in which Afghan forces have been experiencing significant losses. The U.S. military will also need additional resources, such as aircraft and fire support, to ensure that American troops have the resources to fight and survive if needed. Those additional air and artillery units, while helping American ground forces, will also likely be used to help Afghan troops fight off the Taliban. In the first six months of 2017, U.S. aircraft had provided roughly the same amount of airstrikes as they had in 2012, according to Air Force data.

“The minute you put a NATO soldier out anywhere in Afghanistan we have the military responsibility, the moral responsibility to provide four enabling architectures,” the official said.

He described those four resources as:

1.) Command and Control (the ability to talk to troops and see them).

2.) Intelligence, Reconnaissance and Surveillance (drones and reconnaissance planes that search for the enemy).

3.) Fire support (more aircraft, rocket artillery and cannon artillery in case advisers are attacked).

4.) Medical evacuation capabilities (helicopters that can get the wounded from the battlefield to a medical facility within the hour).

The official said the additional troops could be deployed within weeks to months.