Samarra

Samarra exists because the caliph al-Mutasim needed somewhere to put his Turkish soldiers. Their presence in Baghdad had become intolerable to the population, and in 836 he moved the court and the troops upriver to a new site, where successive caliphs then built palaces, barracks, racecourses and mosques along some thirty kilometres of the Tigris. The capital returned to Baghdad after about half a century, and the site was largely abandoned, which is why it survives as the largest archaeological city of the Islamic world and was inscribed on the World Heritage list in 2007.

Its most recognisable building is the Great Mosque of al-Mutawakkil, at its completion the largest mosque anywhere, with a free standing spiral minaret climbed by an external ramp. Nothing else quite like it was built again.

Samarra was also where the state tried to settle a doctrinal question by force and failed. The inquisition begun under al-Mamun required scholars to affirm that the Quran is created rather than eternal, a Mutazili position, and those who refused were imprisoned or beaten. Ahmad ibn Hanbal's refusal made him a hero of the traditionist party. Under al-Mutawakkil the policy was reversed entirely, the Mutazila lost official favour, and the episode is generally read as the moment the caliphate ceded the definition of orthodoxy to the scholars, who have held it since.

The tenth and eleventh Twelver imams, Ali al-Hadi and his son Hasan, were brought to Samarra and kept under supervision near the military cantonment, which is where the name al-Askari comes from. Both died there and are buried in the same shrine, along with the mother of the eleventh imam and the woman regarded as the twelfth's mother.

The doctrine that grew from the events of 874 is what makes the site central. Twelver Shia hold that Hasan al-Askari left a young son, that the son entered concealment to preserve his life, and that for some seventy years he communicated with the community through four successive deputies. When the last deputy died in 941 without naming a successor, the greater occultation began, and the imam remains alive and hidden until he returns as the mahdi to fill the earth with justice. A cellar within the shrine complex is associated with the concealment and is visited accordingly, though the belief itself does not depend on any particular room.

That doctrine shapes Twelver religious authority in the present, because in the imam's absence the qualified jurists exercise functions that would otherwise be his, which is the ground on which modern arguments about clerical government are conducted.

The shrine's golden dome was destroyed by a bomb in February 2006 and its minarets in 2007. The attacks were widely understood as designed to provoke, and they did: the months that followed were the worst of Iraq's sectarian killing. The dome has since been rebuilt.