Yazid instructed his governor, ‘Ubaidullah Ibn Ziyad, that if Husain (as) refused to pledge allegiance, he should fight the Imam (as).
Prior to the incident of Kerbala, ‘Ubaidullah Ibn Ziyad had appointed ‘Umar Ibn Sa`d as governor of the province of Rey, but before he could go there, ‘Ubaidullah Ibn Ziyad sent him a letter, which contained the following instructions: ‘Husain (as) has arrived in Iraq. First proceed to Iraq, fight with him, kill him and then proceed towards Rey.’
‘Umar Ibn Sa’d approached ‘Ubaidullah and requested: “O’ Amir! Relieve me of this responsibility!”
“I shall relieve you of this responsibility but I shall also relieve you of the governorship of Rey,” ‘Ubaidullah responded.
‘Umar Ibn Sa’d found himself hesitating between fighting the Imam (as) and governing the great kingdom of Rey. He requested ‘Ubaidullah to give him respite for a night so that he could reflect over the matter. ‘Ubaidullah agreed and ‘Umar Ibn Sa’d passed the entire night pondering over the matter till he eventually opted for the kingdom of Rey, which lay before him at that moment, but chose to disregard Hell and Paradise, which were in the next life. He decided to fight the Imam (as). The next morning, he went to ‘Ubaidullah and expressed his readiness to accept the responsibility of fighting the Imam (as). ‘Ubaidullah placed a huge army at his disposal so that he could proceed to Kerbala for the purpose.
Imam Husain (as) entered Kerbala on the 2nd of Muharram while ‘Umar Ibn Sa’d, in his capacity as the supreme commander of his army and with a four thousand strong cavalry, arrived there on the 3rd.
He appointed Shimr as the chief of his army and on the 10th of Muharram went to the extent of ordering the killing of Imam Husain (as) and seventy two of his children and companions just for the purpose of acquiring the kingdom of Rey.”( Muntahal A’mal, Volume 1, Page 333 )