Mu'awiyah died. His son Yazeed succeeded him. So, the Caliphate became a kingdom.
Yazeed was a bad man. He drank alcohol. He committed illegal actions. He amused himself with his dogs and monkeys. Thus Imam Husayn [s] refused to pay him homage.
The people in many Muslim cities suffered from Mu'awiyah's unjust policy. They wanted him to die to get rid of his bad behaviour. When the Muslims knew that Yazeed would be the Caliph, they felt pain and were full of anger. They opposed his rule because he did not respect Islam and Muslims.
The people in Kufa liked Imam Ali [a] because he was just and merciful. Besides they knew al-Husayn's good behaviour and his kind treatment towards his Muslim brothers. Therefore, they sent him many letters.
In Makkah, Imam Husayn [s] received the Muslims messengers. In their signed letters, the Muslims said to Imam Husayn [a]:
Come to us! You are our leader only!
Imam Husayn [a] accepted the Muslims request. He decided to revolt against Yazeed bin Mu'awiyah. He took his family and supporters and left for Kufa.