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Christian Era. CE was previously called AD, which stands for anno domini [year of our Lord].
Skipping forward many years, the Prophet Muhammad’s following grew in Medína. The Meccans however would not leave Him alone and keep attacking Him, until eventually He decided that He would end this. The Prophet and his followers came to Mecca, defeated the Meccans and destroyed all the idols in the Ka’ba. And a little while later God revealed to the Prophet Muhammad that the Ka’ba is the point towards which Muslims should turn to pray their daily obligatory prayers. Before this the Muslims would turn to Jerusalem to pray their obligatory prayers. Mecca also became one of the sites of Pilgrimage (Hajj) together with Medína (where the Prophet died and is buried).
Mecca and Medína are the two holiest cities for all Muslims in the world, regardless of which branch of Islam they adhere to. Pilgrimage (Hajj) is ritual obligation for all Muslim men once in their life. Those who have done it, take on the title of Hájí.
Importance of the ‘Atabát (Karbalá’, Najaf, Kázimayn, Sámarrá)
For the Shí’a, in addition to the Hajj, special visitations (zíyárat) to the resting places of the Imáms is also part of their religious obligations. This makes cities where the Imáms and their siblings are buried very significant. Of these sites, four are located relatively close to each other and Baghdád. These four towns/cities form what the Shí’a call the ‘Atabāt-i-’alíyāt or moqaddaseh (meaning lofty or holy thresholds). They are Karbalá’, Najaf, Kázimayn and Sámarrá.
Karbalá’ is where Imám Husayn was martyred and is where he is buried (his tomb is in the centre of the city). Imám Husayn’s brother ‘Abbás is buried in the south-east quarter of the city.
Najaf is about 45 miles south of Karbalá’ and is where Imám ‘Alí is buried. Kúfa, where Imám ‘Alí was killed, is adjacent to Najaf.
Kázimayn is across the river Tigris from old Baghdád and is now essentially a part of Baghdad (most high-level maps don’t show it as a separate town). It is where the 7th and 9th Imáms are buried.
Sámarrá which is about 80 miles north of Baghdád is where the 10th and 11th Imáms are buried. It is also where the 12th Imám goes into occultation.