The Seljuks were members of a Turkish clan from north of the Caspian and Aral seas. At the end of the 10th century AD they migrated to Khwarazm and Transoxania (an area covered by modern Afghanistan, Kazakhstan, Uzbekistan, Turkmenistan, Tajikistan and the Kyrgyz Republic) to serve in the armies of the rulers there. Gradually, they displaced their Ghaznavid overlords in Khurasan and by 1040 the Seljuks had taken over Ghaznavid lands (covering the area of modern Iran, Afghanistan, and northwest India and Pakistan).

In 1055 the Seljuk leader Tughril Beg entered the Abbasid capital Baghdad and proclaimed himself sultan. However, as ruler of the Abbasid caliphate in Iran, Iraq and Syria, Tughril Beg still recognised the spiritual authority of the caliph.

The Seljuks held power for around 200 years through a highly organised government, a system of regional governorships and a professional army. In the late 11th century a branch of the dynasty took control of Anatolia (modern Turkey) and established the sultanate of Rum on what had previously been Byzantine territory. Territories of both the Rum Seljuks and the Great Seljuks fell to the Mongol invaders during the 1250s.