How did Imam Hossein (AS) Save Islam?

Only 50 years after the demise of the Holy Prophet, the leader of the Muslim nation was a man who did not abide by any of the Islamic commandments and openly indulged in sinful behavior. Yazid’s family, the Umayyads, with the leadership of his grandfather, Abu-Sufyan, were the main enemy of the Prophet and had only converted to Islam after Muslims had conquered their home city of Mecca, solely to save their lives. They were not sincere in their belief in Islam, and were always looking for their power and benefits. They had the chance to reach power during the time of the second and third caliphs, and were able to stabilize their power during the long governorship of Muawiyah.
The title that Muslims used for the caliph was literally ‘Successor of the Prophet’, and they also called the caliph the ‘Commander of the Faithful’. Muslims viewed the caliphate as a sacred religious leadership and believed in political obedience despite the character of the person in power. This belief was deeply rooted, to the extent that following the legacy of the previous caliphs was part of their faith, and a requirement for the new caliph. In this situation, any action against the caliph was interpreted as a false action against the unity of the Muslims and publicly condemned.
Imam Hossein knew that he had only two choices, pay allegiance to Yazid, or be killed. The Imam had said, “Even if I hide inside a hole, they will finally find me and kill me if I don’t pay allegiance.” However, the Imam knew that his allegiance would legitimize Yazid as a leader, as well as the Islam propagated by Muawiyah and the Umayyads. Therefore, the Imam pursued the second choice, and sacrificed everything he had for the sake of God and the awakening of the Prophet’s nation. He was martyred in an unprecedented, horrifying, and brutal way, where his family and companions were martyred thirsty, his six month old baby slayed, his head slaughtered, his belongings looted, his ring finger cut off, his body trampled, his family imprisoned, all in the holy month of Muharram, when war and bloodshed was strictly forbidden (Haram) in Islam.
The Imam made a series of wise decisions to maximize the effects of his martyrdom. The Imam’s choice to move from Medina to Mecca, to informing the pilgrims from all over the Islamic territories, accompanying his family (women and children) to witness and carry his message, postponing the battle from the darkness of the night to the daylight of Ashura, and his martyrdom in front of thousands of witnesses, these were all reasons that his message would never be forgotten or misinterpreted.
The Imam’s martyrdom not only disqualified Yazid as a righteous caliph, but for the first time since the Prophet’s demise, put an end to the so-called sacred caliphate system and the Islam that was propagated by those caliphs. Shortly after the Imam’s martyrdom, a chain of objections against Yazid and the Umayyads initiated, which included the battle of al-Harrah in Medina, the