A sunni brother mentioned:
Imam Hussein (A.S) was truthful in his determination to change the munkar, but he was adviced by the likes of Ibn Abbas and Ibn Umar that he was overoptimistic with the people of Iraq and their help. Hussein, being a human who is not ma3soom like the prophets, exercised his Ijtihad.
Based on what is even reported by the Sunnis, Imam Hussein knew that he will be killed in Ashura in Karbala, and yet he went forward. Not only Imam Hussein, but also all the Prophets of God knew about the detail of this catastrophe. From Adam, to Noah, to Moses, to Zakariyya, to Yahya (John), to Muhammad, all cried for Imam Hussein (A.S) and remembered him on the day of Ashura.
Imam Hussein, before he leaves Medina, saw the Messenger of Allah in dream who told him:
"Verily Allah willed to see you martyred. Verily Allah willed to see your family captives"
He knew that he will be killed where and how. There was no error in his part or what you called overestimation of the people of Iraq. A person who goes to a battle which has an uncertain ending, never takes his family, women, and children with him, and never put their lives in to jeopardy. But
Imam Hussein did all that, knowing all the tragic events which will happen to them from setting their tents on fire (by the soldiers of Yazid) to plundering them and moving them from town to town. But he took with him women and children because they were supposed to deliver a message to the people of each town and to wake them up from their deep sleep. No doubt that the event of Karbala was a turning point in the destiny of the religion of Islam, by which the religion was immuned form annihilation. Individuals, however, are not immune from going astray. But the blast in Karbala and its inspiring effects on the hearts of people never let Islam to be eradicated.
Another brother mentioned that if one knows that by going to certain place, he would be certainly killed, the going to that place is considered to be suicide, and as such, how could Imam Imam-Hussein go toward Karbala if he knew that he would be killed?
The above question is interesting. Imam Hussein knew where and how he will be killed and who will kill him. This was the case for the other Imams of Ahl-ul-Bayt as well. Even Sunnis narrated that the Prophet (PBUH) told Imam Ali who kills him and when will he be killed. But none of them avoided what Allah foreordained. Thus Imams having the knowledge of when they die is confirmed by both Sunni and Shia.
It should be noted that the prophets and Imams share with the rest of humanity the means for obtaining knowledge which Allah has given: the senses, the intellect, etc. They also possess special powers and means which other people do not have. In carrying out the commands of Allah in which people have also responsibility, and likewise in ordinary behavior, the prophets and Imams only make use of the first way of knowing, that is the commonly available means. The second means (extraordinary means) is only used by them in duties and works which are connected with their position of prophet hood and Imamate.
Thus in matters such as knowing the beginning of the month, passing judgment, finding out if something is unclean or pure, etc., they make use of ordinary means such as sighting of the moon, and so forth, which anyone else employs. The extraordinary means of knowledge cannot be the basis for their action (unless in some rare circumstances when Allah wishes otherwise), and what they volitionally do must be determined by the means available to everyone. In other words, the act of Imam is based on the conclusion of pieces of evidences which can be obtained by the ordinary human beings, and not by the information related to what Allah has foreordained, otherwise a contradiction would arise: For instance, Allah foreordained that Imam Ali be killed by Ibn Muljam (LA) in the Mosque on a specific date, but since Imam Ali knows this via an extraordinary means of knowledge given by Allah, if he avoids the Mosque, he would have gone against what Allah foreordained which is impossible.
Likewise, Imam Hussein (A.S) though knew most of the people of Iraq will not follow him, he did his duty by answering their unanimous call. Imam Hussein sent his agent, Muslim Ibn Aqeel to Kufa, since he wanted to do the investigation as an ordinary human being does. This is while he knew the situation in Kufa by extraordinary means of knowledge and that Muslim Ibn Aqeel will be killed, yet he was not supposed to act based on what is concealed from an ordinary human being. He acts based on the extraordinary means of knowledge only if Allah wishes so and such circumstances are rare.
Thus, such extraordinary knowledge has spiritual aspect as being the Representative of Allah (Khlifatullah), and the reason for it must be sought on this level, and it is not for the purpose of influencing and controlling the events on the level of ordinary understanding.
source : www.tebyan.net