Newly launched airstrikes in Syria are only one piece of the puzzle in the war against the Islamic State, as the U.S. military prepares to launch a complex operation to train and arm Syrian rebels. 

Just how that operation, formally approved by Congress last week, will play out is largely an open question. But based on past operations including those the U.S. already is running, analysts say allies in the region would likely help in getting military aid to rebels -- whom the U.S. hopes will one day fight as a cohesive unit to rout the Islamic State in their Syria headquarters, aided by airstrikes. 

On Friday, Defense Secretary Chuck Hagel said the U.S. is now "setting up the vetting system" to determine which opposition fighters will get U.S. training. Hagel, while unable to say who the head of that opposition is, said that process would include "regional partners" as well as the State Department and intelligence agencies. 

Lt. Gen. William Mayville, head of operations for the Joint Chiefs of Staff, said earlier this week that the mission is in the "beginnings of implementation" and described it as "a multi-year program." 

This is not the United States' first foray into the region. 

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In the year leading up to Monday’s airstrikes, the CIA had set up camps in Jordan with the purpose of turning Syrian rebels into competent foot soldiers. 

"It is my understanding most of the lethal aid that has been provided by the U.S. has been through the clandestine channel – the CIA," Jeffrey White, a defense fellow at The Washington Institute for Near East Policy, told FoxNews.com. 

While the Pentagon said it could not comment further for this story, White speculated that there likely is a "third party being used to actually deliver" weapons to the Syrian fighters. 

He said the U.S. probably is getting help from Syria’s regional neighbors Jordan, Turkey and Saudi Arabia. It remains unclear what role they might play in the new train-and-equip mission. Reuters reports that Saudi Arabia is among the countries that has offered to host a training facility. 

To date, the U.S. mostly has been sending humanitarian aid into Syria. For that, there is a clear route. 

A July vote from the 15-member U.N. Security Council opened up four routes from Iraq, Turkey and Jordan into Syria. The resolution, though, also approved a monitoring arm that would make sure only humanitarian supplies – like food and medical equipment – would be allowed in the country, effectively closing the path for governments to use those routes for military supplies. 

Earlier this month, Secretary of State John Kerry announced that the U.S. government is providing an additional $500 million in humanitarian assistance to Syria. The funds bring the total U.S. humanitarian figure to more than $2.9 billion. According to the latest numbers from the United States Agency for International Development, or USAID, there are currently 10.8 million people in need of humanitarian assistance in Syria. Of those, 5.5 million are children. USAID is a federal government agency responsible for administering civilian foreign aid.

With the U.S. maintaining no ground forces in Syria, the route for military supplies may be more complicated. 

Still, Syrian opposition fighters have found a way to arm themselves over the course of the three-year civil war. "There’s no question that the rebels are getting weapons," White said, adding it’s nearly impossible to know the exact kind of weapons being fast-tracked to Syria in the wake of the U.S.-led airstrikes.

Sources tell FoxNews.com that U.S.-made weapons were used by rebels during a "test run" earlier this year. In April, a video uploaded by Harakat Hazm, a Syrian splinter with an estimated 7,000 members, showed its fighters using U.S.-made Tube-launched, Optically-tracked, Wire-guided antitank missiles, commonly called TOW missiles.

The use of BGM-71 TOWs, which are capable of cutting through heavy armor, marked the first time U.S.-made anti-tank missiles had appeared in rebel hands. The TOWs were introduced during a time when Syrian forces, using Russian-supplied weapons and ammunition, were quickly gaining ground on the rebels.

In the past, the United States had sold TOW missiles to Turkey. In December, the Pentagon approved the sale of 15,000 TOWs to Saudi Arabia. Though it's unclear how exactly the missiles got to the rebels, Turkey and Saudi Arabia are both U.S. allies and have helped rebel fighters in the past. 

TOWs are used in 40 armies globally and are the “preferred heavy assault anti-armor weapon system for NATO, coalition, United Nations and peacekeeping operations worldwide,” according to Raytheon, the company that makes TOWs. Current versions of the missiles can penetrate more than 30 inches of armor. The missiles can be fired using a tripod or from vehicles and helicopters. 

Going forward, military experts say rocket-propelled grenades and other communication equipment are also on the list of items the United States wants to send to Syrian opposition.  

But arming rebels is only part of the multi-layered solution, they tell FoxNews.com.

Training local fighters may be the toughest battle yet – and several military experts FoxNews.com spoke to say there is no easy path to a post-Assad Syria.

“After three years of inaction, anything the United States does will be in a difficult environment,” Foreign Policy Initiative Executive Director Christopher Griffin told FoxNews.com. “We need to train the trainers. We don’t want to subcontract this to others.”

Just how long will the training take?

By the Pentagon’s own schedule, it could take up to five months to identify trustworthy rebels and then up to a year to train them into an organized militia. Defense officials say they can train 3,000 rebels per year once the program become operational. 

Amid concerns that the ambitious operation could simply take too long -- with ISIS continuing to threaten the entire region -- White claimed that training smaller groups could be done in weeks.

“To produce a basic soldier, it would probably take weeks or a few months depending on how good we want them to be,” White told FoxNews.com. 

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told a Senate hearing that about two-thirds of ISIS’s personnel – which the CIA has estimated to be between 20,000-31,500 – are in Syria.