The Universal Message of Ashura

Husayn (as) was killed on the battlefield as he did Sajdah. His head was removed from his body on the plains of Karbala, mounted on a spear, and paraded through villages and towns as it was taken to Damascus and presented at the feet of Yazid. To the kaffirun he looked, and looks, like a fool. This is because to the temporal-minded man his actions of self-less devotion to the cause of Allah seem fruitless since he gained nothing physically, and lost everything of this world because of devotion and service to Allah.
However, it is more than abundantly clear that Husayn (as) was aware of the dangers that he would certainly encounter in Karbala. Clearly, Husayn (as) had in mind a certain strategy to bring about a revolution in the consciousness of the Ummah. It is clear that Husayn did not try to organize military support, or to even fully exploit the military strength, which was already available to him. In fact, he specifically asked his supporters to abandon him as he headed for Kufa.
All of Imam Husayn's (as) actions demonstrate that he was fully aware of the reality that mere military conquests are temporal victories that do nothing to change the hearts or consciousness of mankind. In time another power will be mustered of greater strength and it will topple the empire before it. Truthfully, in the rising and falling of nations are many signs for those who reflect.
If Husayn (as) would have accepted the iniquitous manner of Yazid's lifestyle then this had been an acceptance of values and norms contrary to the Din of Islam, it would have detrimentally affected the Ummah for all time to come. Yazid's manner of living would have come to prevail and be accepted by Muslims who would have thusly reverted to the wickedness of those who Muhammad (saw) came to and reformed. It would have, in short, undone what Muhammad (saw) and the Imams before Husayn (as) had done.
Husayn (as) was no ordinary man. Like the other Imams and the 14 Infalibles, he was endowed with a perception clearer than those were not of his station. An average person would have turned away when warned. He would not have risked being killed in the manner that Husayn (as) was killed. At the very least, even if the man were a courageous man, he would have taken the advice of his uncle Ibn Abbas and left his family and newborn child behind. Yet Imam Husayn's (as) vision was clear and he strove onward with his family accompanying him.
Aware of the nature of the tyrannical kaffirun pitted against him, Husayn (as) knew that upon his own martyrdom, his family would be captured and taken as prisoners of war, from Kufa to Damascus. This caravan of Muhammad's (saw) own flesh and blood in captivity would publicize the message of Husayn's (as) martyrdom, and ultimately of the path to Hell which the Ummah was being led down by the Umayyads.
Just as Imam Husayn knew, these things happened. As he begged the infidels to spare his infant son, and to permit his son some water to drink (as both women and children among Husayn's people were being deprived of such resources by the Umayyads), these demons shot an arrow through the throat of his son, the great grandchild of the Prophet Muhammad (saw). Husan (as) fought diligently against this horde of devils, but his strength and perseverance was overcome by the shear mass of them. Eventually he was dealt a final blow, and decapitated along with the others among him. Their heads were then placed upon spears and paraded in a procession as the oppressors returned from their battle.
Among the captives taken were Zaynab (as) the sister of Husayn (as) and `Ali ibn Husayn (as), the only surviving son of Imam Husayn (as). As prisoners of war, they were taunted and were it not for Zaynab (as) who protected the young `Ali ibn Husyan (as), then `Ali (as) too would have been killed then and there. As it was, the head of Husayn (as) was struck on the lips with the cane of `Ubaydu'llah. Some of those witnessing the scene were intensely moved by this disrespect and satanic moral indifference. One is recorded as speaking up to say "Remove your cane from those lips, for, by Allah, many times have I seen the lips of the Prophet of Allah on those lips."
Notes :
Al-Mufid, Al-Irshad, pp. 227-8 (Tr. Pp. 364-5) Also quoted in Majlisi, Bihar al Anwar, Vol. 45, p. 116. Some Shi'a histories have a similar episode occurring when the head of Husayn reaches Damascus and is hit by Yazid.
Just how badly the Ummah would have been affected if Imam Husayn (as) had not made this momentous sacrifice is difficult for one to gauge. For the very simple fact that his efforts were ultimately successful, we can only know that the Ummah would have deteriorated and become rotten with corruption far sooner than had. As it is, the Prophet Muhammad (saw) has told us that Islam would deteriorate after the 12 Imams, and specifically would resemble Islam in name only by this current era.
For example, in Sahih Muslim, Jabir says, "I heard the Prophet say, 'Islam will always be dear until twelve Khalifatun and successors." Then he said something, which I did not understand. I asked my father and he said, 'The Prophet said, "They are all from the Quraysh."
Notes : Sahih Muslim, Kitab al-amanah, bab al-mal tih 'l-quraysh
"There Apostle of Allah said: "There will come a time for my people when there will remain nothing of the Qur'an except its outward from and nothing of Islam except its name and they will call themselves by this name even though they are the people furthest from it. Their mosques will be full of people but they will be empty of right guidance. The religious leaders (furqaha) of that day will be the most evil religious leaders under the heavens; sedition and dissension will go out from them and to them will it return." - The Prophet Muhammad (saw)
Notes : Ibn Babuya, Thaqab al-Amal, quoted in Majlisi, Bihar al-Anwar (old ed.), Vol. 13. p. 152