The Battle of Karbala is the story of the movement of Imam al-Husayn (a) from Medina to Mecca and from there to Kufa, that in the middle of the way, the army of ‘Ubayd Allah b. Ziyad led by ‘Umar b. Sa’d lined against the caravan of Imam al-Husayn (a) which led to the battle between them and the martyrdom of Imam (a) and his companions and the capture of his family. This tragedy is among the most heart-breaking tragedies to Muslims and especially Shi’a in the Islamic history

The movement of Imam al-Husayn (a) began from Medina towards Mecca after the death of Mu’awiya on the first days of Rajab 60/679; and it was the first action of Imam al-Husayn (a) to refrain giving allegiance to Yazid b. Mu’awiya. In this journey, the family of Imam (a), some of Banu Hashim and some Shi’a accompanied him

Imam al-Husayn (a) stayed in Mecca until Dhu l-Hijja 8/September 12; but, following the people of Kufa’s request of him to accept governorship and regarding the possibility of being killed in Mecca by the agents of Yazid, Imam (a) moved towards Kufa; however, before arriving in Kufa, he (a) was informed that the people of Kufa had broken their promise to support him. After Hurr b. Yazid al-Riyahi blocked the way of Imam (a), he (a) went toward Karbala and there he (a) encountered the army of ‘Umar b. Sa’d

Source : wikishia