In Iraq the Persians had their capital at Al-Madain. Al-Madain was situated across the Euphrates and the Tigris and was situated on the eastern bank of the Tigris. After the occupation of Al-Madain, the city served as the provincial capital for the Muslims. The climate of the city was damp, malarious and unhealthy for the Arabs used to the dry climate of the desert. When the Muslim officers from al-Madain waited on Umar at Madina he was struck by the fact that such persons by residence in Al-Madain had lost in health. Umar looked into the causes for the decline in the health of the people and he came to the conclusion that that was due to the unhealthy climate of Al-Madain. Hadart Umar accordingly desired that another city should be founded as the capital of Iraq. His instructions were:
(1) Only that site would suit the Arabs as would suit their camels. The new site should be such where the climate is dry.
(2) As far as possible there should be no water between you and me. Choose a site for the new capital which is to this side of the Euphrates and the Tigris.
Salman and Hudaifa were accordingly commissioned to select a new site for the provincial capital. For this purpose they selected the site of Kufa. It was at some distance from the western bank of the Euphrates and the climate was dry. Before Islam, Numan bin Mundhir had his capital here. In the neighborhood there were some buildings of the period of Numan. Arab flowers like Uqehawan, Shaqaiq, Qaisum and Khazami grew here in abundance. The Arabs called the site "Khadd-ul-Azra"-The Beloved's Cheek. The soil was sandy and gravelly and suited the Arab temperament. Umar approved of the site, and because of the sandy and gravely nature of the soil, the place was named Kufa.
Umar ordered that houses should be constructed in the city to accommodate 4O,000 persons. Each Arab tribe to be settled in the city was to have a separate quarter. The town was laid out under the supervision of Hayaj bin Malik Umar gave instructions about the laying out of roads and streets. The main roads were to be 40 cubits wide. The subsidiary roads were to be 30 cubits wide. The streets were to be 2O cubits wide, and the side lanes were to be 7 cubits wide. The Jamia Masjid was constructed in the centre. Adjoining the mosque was the central market. Then a few public buildings were provided of public character such as Government House, the Treasury, the Guest House etc. The town was divided into two dozen quarters. Each quarter was inhabited by one tribe. Each quarter had its own mosque. All houses were to be single storeyed and no house was to contain more than three rooms.
Within a year the new town was completed and the Muslim forces moved from Al-Madain to Kufa. Umar called Kufa, 'the Glory of Islam'.
At the time Kufa was built in Central Iraq a new city was built in Southern Iraq near Uballa a port on the Persian Gulf. Utba bin Ghazwan was commissioned to select a site. He selected the site of Karibah where there were some ruins of ancient times. The land was gravelly. Water and pasture were available. The climate was dry. Umar approved the site and the town was named Basrah. According to one version the town was so named because of the gravels that abounded on the site. According to another version 'Basrah' meant 'Bis Rah' i.e. many roads, and was so named because of its strategic importance.
The city was town planned on lines similar to Kufa. The Friday mosque was provided in the centre. From the central square roads radiated in all directions dividing the city into various quarters. Each quarter was populated by one tribe and each quarter had a mosque.
Originally most of the houses both in Kufa and Basra were of wood. A year after the foundation of the towns these houses caught fire and were burnt. Umar then ordered that houses of bricks should be constructed. It was laid down that no house should consist of more than one storey and no house was to comprise more than three rooms.
In Upper Iraq a town was laid out outside the fort at Mosul. The town of Mosul was laid out under the supervision of Harthama bin Arfaja. The same design as at Kufa and Basra was adopted. At Mosul there was a considerable population of the Christian Arabs and some quarters were exclusively earmarked for the Christian Arabs.
All the three cities namely Kufa, Basras and Mosul rose to great importance. Kufa became the capital of Iraq. Basra rose to importance as a seat of learning. Mosul rose to importance as a trading centre According to a saying that got current at the time while Nishapur was the gateway of the east, and Damascus was the gateway of the west; Mosul was the pathway of the east and the west, for when proceeding from the east to the west or from the west to the cast one had to pass through Mosul.