Ya Hussein!
Each religion has its definitions of sacrifice and martyrdom, and Islam is certainly no exception. But just like other religions that develop a specific vocabulary and belief about those who give up their lives in the name of their god, what martyrdom means in Islam depends on who you ask. For the Shia, who as a group form Islam's second largest sect, martyrdom begins and ends with a central, historical event: the death of the third Imam, Hussein, in the Iraqi desert in 680 CE.
The Shia, as we collectively know them, began life as a political protest over who should succeed the Prophet Muhammad after his death. For some within the young Muslim community, that honor fell onto Muhammad's cousin and son-in-law, Ali, who was married to the Prophet's daughter, Fatima. This group was known as the shiat Ali, or "party of Ali". Feeling that only the descendants of the Prophet (those of his own household) were qualified to lead the community, the Shia felt that the decision of the community to bypass Ali was an act of ursupation. Those Muslims who had agreed by consensus on the next leader, called the caliph, have formed the majority of Muslims in the world today, called Sunnis.
Ali waited thirty years before he finally became the leader of the Muslim community, and his own reign did not last long, ending in his assassination. Since the Shia believed that only the household of the Prophet could properly lead, the eldest of Ali's two sons, Hasan was designated the second Imam. But Hasan was prevented from leading through political maneuvering, so he gave up the role of caliph to an individual named Muawiyyah, who would later found a dynasty by passing on the leadership of the Muslim community to his son, Yazid.
The Shia were not to give up, though. When Muawiyyah was close to dying, the Shia trumpeted Ali's second son, Hussein, as the next successor. There was a power struggle of course, since Yazid wouldn't stand by and let a "pretender" to the proverbial throne take his place. Hussein's own supporters were clamoring for him to take the position of caliph by force, if necessary, and scores of supporters in Kufa (in Iraq) pledged their assistance if Hussein would take Yazid on.
Hussein's own intelligence discounted the stories of mass support coming from Kufa. His cousin warned him not to undertake a journey there on the way to facing Yazid for the caliphate, but Hussein proceeded anyway, taking with him only seventy other souls, including several woman and children, among them his own son.
The reports turned out to be accurate: the mass of supporters that had pledged to back Hussein never materialized from Kufa. Hussein, knowing that he had to play out his role, made his stand against the massive contingent that Yazid had dispatched from Damascus. The superior force met Hussein's small force cut off his water supply before the final engagement began: in the end, Hussein was overwhelmed. His men were all killed, and he himself finally was killed, his head being cut off and sent to Damascus, before it finally wound up in Ashkelon. It was eventually transferred to Cairo during the Crusades.
The sacrifice of Hussein on the battlefield of Karbala has come down to the Shia as a monumental event. It can be said that Hussein's lonely stand epitomizes what it means to sacrifice one's life against overwhelming odds, especially when it's viewed as making a stand against corruption and injustice. Because of this, the martyrdom of Hussein transformed the original Shia political protest into a religious one. It also forced the Shia to accept the political order of things, which is why Shia are often described as being "quietest" and staying out of politics.
Shia Islam is itself divided into sects, but the most well-known is called "Twelver Shiism". After Hussein's death, his son, whose life was spared at Karbala because of illness, became the fourth Imam, and the line of succession followed down throughout the ages until the twelfth Imam disappeared. This individual originally cut himself off from the community and continued to speak through assistants before completely severing the ties between himself and humanity. For this reason, the twelfth Imam is said to be in occultation: he is alive and somewhere on Earth, but for divine reasons, will only return at an appointed time in the future as the mahdi. Only then will the rule of Allah be established and justice fill the earth.
Until then, the community is ruled over by learned jurists in the absence of the twelfth Imam. This by itself, is the cornerstone of the late Ayatollah Khomeini's rule in Iran: only the jurist, steeped in the knowledge of the Quran and Islamic law, could properly lead along with the clerics. The jurist does not replace the hidden Imam, but rather keeps society going until the hidden Imam's reappearance as the Mahdi.
But central to the ideology of the Shia is martyrdom, and suffering in the cause of the oppressed against injustice. During the long war between Iran and Iraq in the years 1980-1988, the drama of an imposed war provided the Iranian government to use the poignancy of Hussein's death to rally a nation. The death of Hussein so many centuries ago, provides a common reference point and greatest example of martyrdom for all Shia. In a way, the symbolism of lonely Hussein fighting a hopeless battle transcends Islamic culture and can find resonance for anyone. But for the Shia, the cry of Ya Hussein (O Hussein!) is a rallying cry and a mournful dirge at the same time.