Three weeks into the allied assault on the ISIS capital of Raqqa, operations to surround the northern Syrian city are complete and street-by-street fighting has begun in the outskirts.

The struggle to deny terrorists the symbolic capital of their self-proclaimed caliphate is likely to be hard-fought, bloody and prolonged. It pits an unknown number of fighters from Arab militias, Syrian Democratic Forces and an anti-ISIS coalition of mainly Kurdish soldiers newly-armed and advised by U.S. Special Operations troops.

They face entrenched ISIS fighters estimated as at least 2,500 by the Institute for the Study of War.   

For months the city’s urban core has been the target of artillery and aerial bombardments by Russian, Syrian and allied forces with hardships inflicted on the remaining civilian population of between 50,000 to 100,000 held hostage by ISIS. While thousands have fled, those captured by ISIS are often executed or used as human shields.

Despite air attacks, ISIS forces are believed to have evacuated across the Euphrates River to the south its senior leadership and considerable equipment and supplies, including its elaborate media operations and chemical weapons.

As it has in the ongoing and unexpectedly prolonged battle for Mosul in Iraq, ISIS is defending with a combination of earthen works, tunnels, homicide bombers, intermittent attacks by cells left among civilians and has laced the ruins and ancient narrow streets with IEDs.

The struggle is further complicated by the nearby presence of Syrian forces loyal to the regime of President Bashir al-Assad and large numbers of Iranian militia and troops using the Syrian civil war as a real-life tactical training ground for future regional adventures under the tutelage of Russian advisers.

The anti-ISIS alliance has its own internal challenges. It includes Turkey, for instance, which regards its Kurdish allies as terrorists. The coalition itself is also riven with other bitter ethnic, political and centuries-old religious rifts that make any future governance problematic, even if the most basic power and sanitary infrastructures had not been destroyed in the struggle to save the city.

Experts expect the Raqqa battle to last longer than currently expected and the aftermath to still involve ISIS raids and suicide attacks from isolated havens. “The fall of Raqqa City,” wrote the Institute, “will be symbolic. But it will not be decisive.”