United States-backed Syrian Kurdish and Arab forces have launched their offensive to unseat the Islamic State (ISIS) in Iraq and Syria from its “capital” in Raqqa, Syria, the anti-ISIS coalition task force announced Tuesday.

The fight for Raqqa will be “long and difficult,” said Army Lt. Gen. Stephen Townsend, the commander of Operation Inherent Resolve-Combined Joint Task Force.

However, he said the offensive would deliver a “decisive blow” to the idea that ISIS has a physical caliphate.

“It’s hard to convince new recruits that ISIS is a winning cause when they just lost their twin ‘capitals’ in both Iraq and Syria,” he said.

Townsend tied the Raqqa offensive to a recent attack in Manchester, England, where a lone wolf attacker blew himself up at a concert with young attendees, many of them young girls.

“We all saw the heinous attack in Manchester, England,” said Townsend. “ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand.”

The Raqqa offensive comes as U.S.-backed Iraqi forces have ISIS surrounded in western Mosul, the terrorist group’s stronghold in Iraq. The coalition’s task force called the fighting “brutal, close-quarters urban combat.”

Coalition forces are expected to support the offensive by advising and assisting local forces, providing equipment, training, intelligence and logistics support, precision fires, and “battlefield advice.”

Currently, about 500 U.S. troops are authorized for deployment in Syria, some of whom are embedded with Syrian Kurdish forces, known as the Syrian Democratic Forces, and their Arab component, known as the Syrian Arab Coalition.

The coalition said the SDF has encouraged civilians to leave Raqqa so they do not become trapped, used as human shields, or targets for ISIS snipers – tactics the group has increasingly used in Mosul as it was surrounded.

After Raqqa is liberated, the SDF will turn the city over to a representative body of local civilians who will provide security and governance, the coalition said.

Townsend said after Mosul and Raqqa are retaken, there will still be a lot of “hard fighting” ahead.

“But this Coalition is strong and committed to the complete annihilation of ISIS in both Iraq and Syria,” he said.

The Raqqa offensive also comes on the heels of a recent terrorist attack in London, where three radical Islamic jihadists in a van mowed down pedestrians on the London Bridge, then got out and started attacking bystanders with knives.