Yazid's succession to Mu’awiya and the letters sent to al‑Husayn
After the destruction of Hujr, the Shi'a in al‑Kufa took al‑Husayn's advice and went underground. It has already been noted that by demanding the cursing of 'Ali, Mu'awiya was breaking the spirit of the concord he had reached with al‑Hasan. He was to break another article of that agreement by appointing his son Yazid as his successor. Al‑Husayn resolutely refused to pledge allegiance to Yazid. After Mu'awiya's death, al‑Husayn left Medina and went to Mecca in order to avoid paying homage to Yazid1.
The Shi'a in Iraq, particularly al‑Kufa, saw this as an opportunity to escape from Syrian domination and give back to al‑Kufa and themselves some of the privileges they had enjoyed under 'Ali. They wrote to al‑Husayn urging him to lead a revolt against Yazid. The support from the Shi'a rank and file must have been increased quite considerably. For not only did al‑Husayn receive letters from the ordinary Shi'a, there were also letters from several of the tribal leaders2.
It seems that these men seeing that the mood of the Kufans was favourable to the Shi'a and al‑Husayn, had decided to try to safeguard their own tribal positions by joining the movement in favour of al‑Husayn. Al‑Husayn decided to act prudently by sending his cousin Muslim b. 'Aqil to al‑Kufa to assess the actual situation there.