The effects of the brutal murder of al‑Husayn have had a profound influence on Islam. To all Muslims it is the most distasteful and abhorrent event in Islamic history. To them and particularly to Shia it represents, even more than that ‑ the acme of self-denial and sacrifice by the family of the Prophet for the Islamic community.
From the accounts, it is clear that the one concession that al‑Husayn was never prepared to make was to pledge allegiance to Yazid. Earlier al‑Hasan had warned
Mu'awiya that he would never make al‑Husayn pledge allegiance to him. Mu'awiya appears to have heeded this advice and avoided the confrontation that Yazid had precipitated. The death of al‑Husayn also marks the beginning of a division within the Shi'a. That group that was later to be known as the Imamis emphasized the spiritual importance of the Imamate and gave up attempts to win political power. This policy may well have been laid down by al‑Husayn's son 'Ali after his experiences at Karbala'. However the doctrine of the spiritual importance of the imamate can already be discerned in 'Abd Allah b. Ja'far b. Abi Talib's letter to al‑Husayn warning him not to go to al‑Kufa:
"If you are destroyed now, the light of the land will be put out. You are the guiding symbol of the rightly‑guided, the hope of the believers."
'Ali b. Al‑Husayn was to follow the quiescent policy of religious leadership. It may have been him who laid great emphasis in the Shi'i adhan, or call to prayer, of the words: Hayya ala khayr al‑amal, "Come to the best of works"1.
Although these words are included in the Zaidi and Isma'ili adhan, it was the lmami‑Shi'is in the later years of the first centuries of Islam, who were to give these words their true significance by their deliberate disavowal of political action.
Thus the martyrdom of al‑Husayn has had profound effects on the history of Islam and particularly the history of the Shi'a.