-- U.S.-backed Kurdish and Arab rebel forces have begun the offensive to retake ISIS' self-proclaimed capital of Raqqa, Syria.
The force of 55,000 fighters known as the Syrian Democratic Forces (SDF) began its initial operations to surround the city in November.
Since they they have pushed towards the city from various directions to close off any potential escape routes for ISIS fighters.
By early Tuesday morning, SDF forces east of Raqqa had already retaken 10 square kilometers of territory on the outskirts of the city.
Seized by ISIS in January 2014, Raqqa became the group’s de facto capital in Syria and a magnet for foreign fighters. City residents have had to endure a brutal regime as the group routinely carries out public executions to enforce its oppressive rules.
The city became the center of ISIS' overseas terrorism plotting, with many attacks in Western Europe tied to Raqqa. That is one reason the top U.S. military commander in Iraq said in November that it was "imperative" to move on Raqqa quickly to head off any current terrorism plans on Western targets.
Lt. Gen. Steve Townsend, the senior commander of Operation Inherent Resolve, reaffirmed that ongoing threat citing the recent terror attacks in England.
"We all saw the heinous attack in Manchester, England,” said Townsend in a coalition statement. “ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand."
Here's a closer look at Raqqa and the military offensive to retake the city.
The city of Raqqa, also known as al-Raqqah and ar-Raqqah, is in northern Syria on the banks of the Euphrates River, about 50 miles south of the border with Turkey. Before the start of the civil war in Syria, the provincial capital was believed to have a population of 220,000.
The mostly Sunni Arab city had minority populations of Alawites and Christians that fled the city after ISIS seized control in early 2014.
Raqqa saw an influx of foreign fighters, ISIS supporters and their families drawn to the capital of ISIS' self-proclaimed caliphate.
The residents that remained have endured oppressive rules on dress and a ban on foreign contact that are enforced with public executions and lashings.
The brutality of life in the city has been documented by a secret group of activists called Raqqa Is Being Slaughtered Silently.
The high-profile terrorist attacks in Paris, Brussels and Turkey that were carried out by ISIS sympathizers originated in Raqqa, according to intelligence officials.
Given its importance in ISIS' infrastructure, the city has often been the target of coalition airstrikes targeting key ISIS facilities and multiple ISIS leaders, including Jihadi John, who was killed by a drone strike in November 2015, according to U.S. officials.
In advance of the offensive, the coalition believes that ISIS leaders have fled the city, as have the group's administrative bureaucracy and media operation, to ISIS-held areas in the Euphrates River Valley. An estimated 4,000 ISIS fighters remain in the city, according to a Pentagon spokesman.
Who Will Conduct the Offensive Against Raqqa?
The Syrian Democratic Forces now number 55,000 fighters, and while still identified as a Kurdish-led organization, the number of Arab forces within the group now number 29,000.
Operating in eastern and northern Syria the SDF has become the most reliable coalition partner fighting ISIS in Syria, taking back a swath of northern Syria, including the cities of Kobani, Manbij, Jarabulus and Taqba from ISIS.
There are more than 900 American troops in Syria assisting and advising the Syrian Democratic Forces. U.S. troops are not supposed to be on the front lines, but many are embedded with the other forces and could be placed in combat environments, according to U.S. officials.
In addition to coalition airstrikes, the SDF forces will also receive support for their ground offensive on Raqqa from a U.S. Marine artillery unit and U.S. Army Apache attack helicopters.
Last week SDF forces also received the first shipments of U.S. small arms, ammunition and vehicles to be used for an offensive on Raqqa, a move that has been opposed by Turkey, which sees the Kurdish forces in the Syrian Democratic Forces as an extension of a Turkish terrorist group known as the PKK.
Gen. Townsend anticipated in a coalition statement issue Tuesday that the fight for Raqqa "will be long and difficult" but that it would deal a decisive blow to ISIS.
Much as happened in Mosul, ISIS has had ample time to prepare defenses against an offensive and U.S. military officials expect Raqqa to be a lengthy operation. Set in stages the Iraqi military’s operation to retake Mosul has already gone on for eight months.