As Shi'ites, many Persians had long felt mistreated by the Arabic Sunni Muslims who controlled the Caliphate for centuries. When the power of the caliphs faltered in the 10th-11th centuries, and Christian Crusaders began to harry them in the eastern Mediterranean, the Shi'a thought their moment had come.
However, a new menace arose to the east, in the form of the newly-converted Turks. Fervent in their beliefs, and militarily powerful, the Sunni Seljuks took control of a vast region including Persia. Outnumbered, the Nizari Shi'a could not defeat them in open battle.
From a series of mountaintop fortresses in Persia and Syria, however, they could assassinate Seljuk leaders - and strike fear into their allies.
In 1219, the ruler of Khwarezm, in what is now Uzbekistan, made a huge mistake. He had a group of Mongol traders murdered in his city. Genghis Khan was furious at this affront, and led his army into Central Asia to punish Khwarezm. Prudently, the leader of the Assassins pledged loyalty to the Mongols at that time.
