With the fall of Aleppo, more than 80,000 mainstream rebels across Syria are at a strategic crossroads, facing three broad choices: go underground, join multinational operations against the Islamic State, or fight on under the ranks of hardline jihadist groups. 

No matter which path the rebels take, the fall of Aleppo ushers in a new phase in the Syrian war, rebels and analysts agree. 

Turkey, Jordan, and the United States are pushing Syrian rebels to evacuate to de-facto buffer zones along the Turkish and Jordanian borders.  Their addition to anti-Islamic State operations would be welcome in diversifying the Syrian Democratic Forces currently advancing on the Islamic State capital of Raqqa. 

Currently, the SDF is 80 percent Kurdish, and therefore unsuited to occupying Sunni Raqqa – a major stumbling block in the US-led coalition’s drive to defeat the Islamic State. 

But the other rebel options are far less palatable.  A full-blown Syrian insurgency could be unpredictable and intractable, while large-scale defections to jihadist groups would in some ways mark a worst-case scenario. 

What seems certain is that, despite its victory in Aleppo, the Syrian government is unlikely to place the entire country under its control “in the near-term, if ever,” says Christopher Phillips, a Syria expert at Queen Mary University in London. 

In that way, the rebels’ decisions will play a significant role in shaping how the Syrian conflict evolves. 

Option 1: Insurgency

In addition to the some 7,000 fighters that were within Aleppo, there are an estimated 40,000 fighters affiliated with the Free Syrian Army outside Aleppo, 30,000 in southern Syria, and several thousand in the suburbs of Damascus and Idlib. 

Rather than attempting a decisive showdown in another Syrian city, many fighters are now likely to abandon their territories and bear down for a new phase in their uprising, some say. 

“We are preparing our units for an underground, insurgency phase – a popular resistance – against the regime [of Syrian President Bashar al-Assad], its allies, and their interests,” says Abdul Hadi Sari, a former air force general and commander in the Southern Front forces. 

According to the insurgency plan, detailed by rebel commanders in the north and south, rebels would meld into local populations falling under regime control and carry out a series of bombings and ambushes of government checkpoints, administrative centers, and other interests. 

The proposed shift in tactics comes out of the belief that a drawn-out insurgency – rather than traditional warfare – would wear down Syrian backers such as Russia and Iran, who would be wary of a several-year deployment in Syria. 

But there is also the recognition that the areas liberated and secured by mainstream rebels have suffered the most during the war. 

“Every time we capture and seize territory, the civilians pay the price.  The regime, Russia, and militias lay siege to them, starve them, gas them and bomb them mercilessly from above,” Mr. Sari says. “ There is a growing sense that holding territory is a liability, not an asset.” 

The shift to an insurgency has tactical merits.  Even with Aleppo’s fall, the Assad regime controls less than one-third of Syria’s territory.  The depleted Syrian Army is also facing a shortage in manpower, reportedly resorting to forcibly recruiting citizens in reconquered territories. 

The fact that the Islamic State retook Palmyra this week demonstrates regime’s vulnerability, rebels say.  Moreover, the rebels have the advantage of being locals. 

“If we cannot confront the regime without [antiaircraft weapons] and without airpower, then we will confront the regime as secret cells,” says Abu Mohammed, a commander of the Free Syrian Army umbrella group Fateh Aleppo. 

Option 2: Fight ISIS

The most obvious option facing rebels, analysts say, would to head northwest into the buffer-zone established by their main backer, Turkey. 

Under this scenario, tens of thousands of mainstream Syrian rebels would join Turkey’s Euphrates Shield operation to drive the Islamic State and Kurdish militias aligned against Turkey from much of northern Syria. 

By joining anti-Islamic State operations, rebels see an opportunity to gain relevance once again and to become indispensable to the global powers that have abandoned them. 

Under this option, rebels would join operations to liberate Al Bab, 30miles northeast of Aleppo, which borders regime-controlled territory.  Taking Al Bab from the Islamic State would pave the way for the Sunni rebel and Turkish forces to push east to Raqqa. 

“In addition to protection, a Turkish-led force liberating Raqqa could embolden the rebels and give them a foothold just when they are pushed out” of Aleppo, says Mr. Phillips of Queen Mary University. 

Already, a reported 10,000 fighters are taking part in Euphrates Shield –many of them from Aleppo. 

Rebels in southern Syria could take a similar course.  They have been in a de-facto cease-fire with the Assad regime for most of the year.  Their task would be to eliminate a small Islamic State affiliate, the Khalid ibn al-Walid Army, near the Golan Heights. 

Under a proposal being pushed by Jordanian officials, Jordanian forces would grant Syrian rebels safe haven along the Jordan-Syria border in return for eliminating the Islamic State and keeping the largest jihadist group, Jabhat Fateh al-Sham, at bay. 

Reportedly, 10,000 Syrian rebels in the south have already signed on to this plan. 

Option 3: Join the jihadis

But some might choose to join Jabhat Fateh al-Sham itself.  Formerly known as the Nusra Front, the group numbers between 6,000 and 10,000 fighters and has long been the most effective and disciplined fighting force in Syria, setting it apart from the often-divided mainstream rebels. 

Although few of the moderate rebels – or Syrian civilians – share Nusra’s militant jihadist ideology, the group has an allure. 

While other options mean being coopted by foreign governments to fight the Islamic State or Kurdish militias, Nusra remains solely committed to fighting the Assad regime. 

Thousands of rebels who wish to continue fighting Assad’s rule will have no other option than to join the ranks of Nusra, which maintains close ties with Al Qaeda. 

“There are some individuals that are drawn to Al Qaeda because they are disappointed with the West and American policy in Syria,” says Fabrice Balanche, Syria expert and visiting fellow at the Washington Institute for Near East Policy. “ If they want to continue the fight, they will join Al Qaeda.” 

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Free Syrian Army-aligned rebel commanders report that some 300 fighters have already broken ranks to join Al Qaeda-aligned militias in Idlib. 

“Aleppo’s fall leaves very little choice for the groups struggling to survive,” says Jennifer Cafarella, an analyst at the Institute for the Study of War in Washington. “ One of the tragedies of the fall of Aleppo will be it will prove Al Qaeda right.”