As US-backed Arab and Kurdish forces broke into the Islamic State groups bastion of Raqa on Tuesday for the first time, here are five facts about the city in northern Syria:

Ancient capital

In an area inhabited since antiquity, Raqa reached its heights under the Abbasid caliphate. From 796 to 809, the powerful caliph Harun al-Rashid transferred the capital of his empire from Baghdad to Raqa, which sat at the crossroads of key trade routes. Major works were ordered and the city was dotted with grand palaces and mosques.

The caliph's court returned to Baghdad in 809 but Raqa remained a major administrative centre for the western part of the empire. In 1258, the city was destroyed by the Mongol invasion.

Strategic location

It is east of Syria's second city Aleppo, 90 kilometres (55 miles) south of the Turkish border, and less than 200 kilometres (125 miles) from the border with Iraq.

Raqa previously prospered from agriculture in the fertile river valley and benefited from nearby hydroelectric dams that generated power for much of Syria.

First major city to fall

Two years after the start of Syria's civil war in 2011, Raqa was the first provincial capital to fall to rebels, among them the Al-Nusra Front, Al-Qaeda's then-affiliate in the country.

But tensions soon erupted into clashes between Al-Nusra and fellow jihadists of a precursor of the Islamic State group.

On January 6, 2014, fierce fighting between the rival groups ended when IS's predecessor seized control of the city.

Five months later, Mosul in Iraq fell to the jihadists and on June 29, IS chief Abu Bakr al-Baghdadi proclaimed a "caliphate" that covered territory in both countries.

IS bastion, gruesome theatre

Raqa became a key city in IS's self-declared caliphate, a hub for the organisation's activities and attacks in Syria, Iraq and further afield.

IS took over all levels of civil administration, rewriting school curriculums, establishing Islamic courts and creating police units to implement Islamic law.

Raqa also became the scene of some of IS's worst atrocities, including gruesome executions, public displays of bodies and sex trafficking.

Coalition target

The city has long been seen as a prize by multiple parties to the Syrian conflict, including the government, Russia, Turkey and the US-led coalition bombing IS since 2014.

On November 5 last year, the Syrian Democratic Forces (SDF), a US-backed Arab-Kurdish alliance, launched a major offensive dubbed "Wrath of the Euphrates" to take Raqa.

An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 who fled there from other parts of the country.

But as the SDF drew closer to the city, thousands were smuggled out to territory newly captured by the US-backed fighters.

After a string of victories in the surrounding province, including the key town of Tabqa and the adjacent dam, the SDF has sealed off the approaches to Raqa city from the north, east and west.

On Tuesday, the SDF said its forces entered the city from the eastern district of Al-Meshleb.

The US-led coalition has been backing the SDF with air strikes, equipment and special forces advisers.

On Monday, a coalition bombing raid killed 21 civilians as they tried to escape Raqa by dinghy on the Euphrates River, a route also used by IS fighters, the Syrian Observatory for Human Rights said.