Mu'awiyah bin Abu Sufyan died. His son Yazeed succeeded him. Yazeed was a young man aged 30. He drank alcohol. He amused himself with the dogs and the monkeys.
So, Imam Husayn [a] refused to pay Yazeed homage. Meanwhile, the Kufians were tired of Mu'awiyah's persecution. Thus, they sent Imam Husayn [a] many letters. In their letters, they asked the Imam to come to save them from the Umayyad persecution.
The spies told Yazeed about the situation in Kufa. Yazeed had a spiteful Christian doctor called Sergon. He asked the advice of the doctor. Sergon advised him to appoint Ubaidullah bin Ziyad a ruler over Kufa.