Samarra Shrine One of Shiite Islam's Holiest Sites

Iraq's Sunni city of Samarra, which Sunni militants are preparing to attack for the third time in a week, houses one of Shiite Islam's holiest sites, the thousand-year-old Imam al-Askari Mosque.

Exactly seven years ago, suspected Sunni insurgents blew up the mosque's two minarets, 15 months after the destruction by an al-Qaida bombing of the shrine's golden dome ignited brutal sectarian clashes across Iraq.

The massive golden dome, about 20 meters (66 feet) high and with a diameter of 68 meters, was covered in 72,000 gold pieces and surrounded by walls of light blue tiles.

Its destruction on February 22, 2006 triggered widespread sectarian bloodshed.

On Friday, top Shiite cleric Grand Ayatollah Ali al-Sistani called for Iraqis to take up arms against "terrorists" of the Sunni Islamic State of Iraq and the Levant (ISIL), led by Samarra native Abu Bakr al-Baghdadi.

The city of around 200,000 people lies 110 kilometers (70 miles) north of Baghdad on the east bank of the Tigris river, and was founded by the Assyrian King Sennacherib, who built a fortress there in 690 BC.

At the beginning of the ninth century, Samarra was still nothing more than a Christian monastery surrounded by a few houses, when the Caliph al-Mutassim chose it for his capital because Baghdad was in revolt.

Both warrior and architect, he brought artisans and materials from across the Muslim world to build Samarra, earning the admiration of his contemporaries.

He called the new town "Surra man ra'a," meaning "Happy is he who sees it", but it was the name Samarra that has endured through the ages. The seven caliphs who succeeded al-Mutassim continued to build on his achievement.

But in 892 Samarra's influence waned with the return to power of Baghdad. Ravaged by Mongol raiders, it nonetheless retained its importance as a place of Shiite pilgrimage.

A double mausoleum was built on the site of the house where the 10th and 11th Imams, Ali al-Hadi and his son, Hassan al-Askari, descendants of the Muslim Prophet Mohammed, were buried.

Most Shiites believe that al-Mahdi, a hidden 12th Imam who disappeared at a young age at around the same time, will one day reveal himself in Samarra as the champion of the righteous in a battle at the end of world.