Yazid I
The Shia regard Husayn as a martyr ("shahid"), and count him as an Imam from the Ahl al-Bayt. They view Husayn as the defender of Islam from annihilation at the hands of Yazid I. Husayn is the last imam following Ali whom all Shiah sub-branches mutually recognize. The Battle of Karbala is often cited as the definitive break between the Shia and Sunni sects of Islam, and is commemorated each year by Shiah Muslims on the Day of Ashura.