In the month of Rajab, 60 A.H., Mu'awiyah died. He had ruled the Muslim countries for twenty years. He killed many companions of our Master Muhammad [s]. Hajar bin Ady al-Kindy, Amr bin al-Hamq al-Khazay and Rasheed al-Hajry were of them. He poisoned Imam Hasan (the Prophet's grandson), Malik al-Ashtar, Saad bin Abu Waqas and others.
In spite of the Muslim's dissatisfaction, Mu'awiyah appointed his son Yazeed caliph. So, the Caliphate became a kingdom.
The Muslims were angry with Yazeed, for he was a corrupt young man. Besides, he drank alcohol and amused himself with monkeys and dogs.
The Muslims in Kufa hoped that Imam Husayn would be the Caliph, for he was our Master Muhammad's grandson. Moreover, he was a good, God-fearing and believing man. And he treated the poor kindly.
So the Muslims sent Imam Husayn hundreds of letters. In their letters, they asked him to save them from persecution.
Imam Husayn [a] was in al-Madina al-Munawwara. He did not pay homage to Yazeed, for the latter behaved badly. In the meantime, he sent his cousin Muslim bin Aqeel as his envoy to Kufa.
Imam Husayn asked his cousin to stay with the most loyal person in Kufa.