The Syrian Democratic Force (SDF) said in a statement they had established a joint operations center Saturday night for the military campaign "Euphrates Rage," an attempt by the US-backed coalition to strike at the heart of ISIS.
Coalition spokesman Col. John Dorrian told CNN that rebel forces allied to the US will begin by working positions east of Raqqa, to shut down the so-called "back door escape" out of both Mosul and Raqqa.
"They have begun their march toward Raqqa," he said.
While Mosul is symbolic as ISIS' last major possession in Iraq -- and the city from which ruler Abu Bakr al Baghdadi announced the creation of his caliphate -- Raqqa holds as much significance for the terror group.
"The importance of Raqqa is that is where ISIS plans their external (terrorist) operations," Gen. Joseph Votel, the commander of US Central Command told
CNN national security analyst Peter Bergen. It is the largest Syrian city still under ISIS control -- and first captured by the militant group.
"Raqqa is recognized as the financial, leadership and external ops center of the Islamic State, so that's what makes it important."
Raqqa is home to nearly 200,000 people, most Sunni Arabs, and an estimated 5,000 militants, according to
the group Raqqa is Being Slaughtered Silently (RSS), one of the only sources of information to come out of the city.
Who is taking the fight to ISIS in Raqqa?
The biggest challenge is finding troops to bring the fight on the ground against ISIS. Most analysts believe that Raqqa will present a different set of challenges from the Mosul campaign because of the absence of local troops able to carry out the assault.
No indigenous allied force currently exists near Raqqa, although the SDF, which has consisted primarily of ethnically Kurdish troops -- primarily Kurdish militias with strong ties to the Kurdistan Workers' Party, or PKK -- has expanded its ranks to include a wider coalition of Syrian rebel groups.
The group, which is seen as largely synonymous with the Kurdish YPG, is viewed with suspicion by the Sunni-majority residents of Raqqa, who are concerned that a SDF victory in their city could lead to forced displacements.
"The SDF is the partner force most capable of acting soon to isolate Raqqah and commenced movement toward Raqqah on 5 November," a statement from the Coalition Joint Task Force-Operation Inherent Resolve said, using an alternate spelling for the Syrian city.
"We believe the inclusion of fighters from the local population is an important advantage to the SDF."
The SDF includes multiple Arab, Kurdish and Turkmen militia groups and will be carried out in coordination with coalition forces, including airstrikes.
US Defense Secretary Ash Carter stressed the need for local forces to take the lead, saying, "We want a victory that sticks everywhere, so it's always local forces."
A staunchly secular Kurdish militia in Syria known as the People's Protection Units, or YPG, has proven to be one of ISIS' deadliest enemies on the ground in Syria.
The Kurds' stiff resistance to the ISIS siege of the border town of Kobane in 2014 prompted the US to send help in the form of airstrikes and weapons.
The Kurdish faction has since succeeded in capturing large swaths of territory from ISIS.
At alternating times the Kurdish militia also has periodically clashed with, and cooperated with, Syrian rebel groups and the Syrian government.
In Syria, up to 300 US Special Operations Forces advising the SDF are authorized to be in the country.
How will the fight for Raqqa complement Mosul operations?
The two cities -- ISIS' last remaining Iraqi stronghold and its proclaimed capital in northern Syria -- are around 360 km (224 miles) apart. The supply line between the two is vital to the jihadists' continued defense of the territory it holds.
Already, Shia militia involved in the offensive around Mosul have cut access
to the main road leading from Mosul to western Iraq and toward Syria.
"It will be some time before they reach the city. In the meantime we will continue shaping operations like airstrikes against Da'esh leaders, command and control and resources. This is the first step in a campaign that will be conducted deliberately," Votel added, using another term for ISIS.
"Doing operations simultaneously with the campaign to liberate Mosul will complicate command and control for the enemy, giving them more problems to solve than their flagging command and control can manage," Votel said.
While the city of Raqqa is smaller than Mosul, the battle to retake it is expected to be fierce. It's ISIS' heartland and also sits in the middle of the country's ongoing civil war, which includes foreign actors such as the US and Russia.
The campaign will proceed in "very deliberate phases," the first of which will be an "isolation phase," according to Special Presidential Envoy for the Global Coalition to Counter ISIL Brett McGurk.
Coalition planes will provide air support while ground operations are to be undertaken by local militia under the SDF banner.
Presumably, the Syrian government -- which claims to want to reunite the fractured country under its authority -- would lead a proposed offensive against a nihilistic movement like ISIS.
However, the government of Bashar al-Assad has been at war with his own people for five straight years.
Throughout this conflict, the military has suffered untold casualties and large-scale desertions.
The secretary of defense also ruled out Russia's participation in the fight for Raqqa, saying that Moscow "is not a participant in our Raqqa plan."
The Russian-Syrian government alliance has appeared far more focused on battling other Syrian rebel groups across the country. These allies have been laying siege for two months to the rebel-controlled part of the Syrian city of Aleppo.
What is Turkey's role?
The rise of the Kurds has also become a complicating factor. Many in Turkey see the establishment of Kurdish "statelets" in northern Syria as a major threat.
The YPG have close links to the Kurdistan Workers' Party, or PKK, Kurdish separatist rebels who have waged a guerrilla war against the state in Turkey for thirty years.
Turkey has declared itself an enemy of ISIS. Turkish warplanes periodically bomb ISIS targets in Syria, while also arresting suspected ISIS operatives in Turkey.
The SDF alliance also includes the YPG, which Turkey considers a terrorist group.
However, Turkey also routinely attacks the YPG in Syria, arguing that the Kurdish militia is as much a threat to Turkish national security as ISIS.
The Turkish-backed Free Syrian Army (FSA) have, in addition to making gains against ISIS, been clashing against Kurdish groups -- including the SDF and YPG.