In order to learn more clearly as to how far the views expressed by Imam Husayn about Yazid are indisputable according to the history of Islam, one should refer to the following remarks of Mas'udi in respect of Yazid.
"Yazid was a pleasure-seeking person. He was a man, who kept beasts of prey. He had dogs, monkeys and panthers. He always arranged wine-drinking parties. One day after the martyrdom of Imam Husayn while he was sitting in such a party, Ibn Ziyad was seated on his right side,Yazid turned to the cupbearer and recited two couplets which are translated below:
"Give me a cup of wine which should satiate my bones. Then give another such cup to Ibn Ziyad, the man who is my confidant and who has strengthened my position and the foundation of my caliphate".
Yazid meant to say that Ibn Ziyad by killing Imam Husayn had strengthened the foundation of his caliphate.
Thereafter Mas'udi writes about the injustice and oppressions committed by Yazid and then says: "In the Muslim ummah Yazid was like Pharaoh amongst his subjects" and then writes: "It is not so, because Pharaoh was more just to his subjects". He adds: "The injustice, intrepidity and impiety of Yazid also penetrated into the Muslim ummah". The sins committed by Yazid were also committed by his favorites and they adopted his ways and manners. During the period of his caliphate music became current in Makkah and Madina, and all sorts of amusements and funs began to be used. The people began to drink wine openly. It is also the more surprising that the man, who claimed to be the successor to the Holy Prophet and occupied the seat of the caliphate, had a pet monkey, which was named Abu Qays. Yazid used to bring it in his drinking parties and he spread a mattress for it. He also mounted it on the back of a she-ass, and made it participate in the horse-race. One day Abu Qays won the race. The monkey was then dressed in red and yellow silken clothes and cloak, and an embroidered cap was placed on its head".
This was the import of the sentence that Imam Husayn wrote to Mu'awiya about Yazid. However, this person attained to the Islamic Caliphate and pressed Imam Husayn to take the oath of allegiance i.e. to acknowledge him to be the lawful successor of his (i.e. Imam Husayn's) grandfather, the Prophet of Allah.