The Battle of Karbala (Arabic: مَعْرَكَة كَرْبَلَاء, romanized: Maʿrakat Karbalāʾ) was fought on 10 October 680 (10 Muharram in the year 61 AH of the Islamic calendar) between the army of the second Umayyad caliph Yazid I (r. 680–683) and a small army led by Husayn ibn Ali, the grandson of the Islamic prophet Muhammad, at Karbala, Sawad (modern-day southern Iraq).

Prior to his death, the Umayyad caliph Mu'awiya I (r. 661–680) had nominated his son Yazid as his successor. Yazid's nomination was contested by the sons of a few prominent companions of Muhammad, including Husayn, son of the fourth caliph Ali, and Abd Allah ibn al-Zubayr, son of Zubayr ibn al-Awwam. Upon Mu'awiya's death in 680, Yazid demanded allegiance from Husayn and other dissidents. Husayn did not give allegiance and traveled to Mecca. The people of Kufa, an Iraqi garrison town and the center of Ali's caliphate, were averse to the Syria-based Umayyad caliphs and had a long-standing attachment to the house of Ali. They proposed Husayn overthrow the Umayyads. On Husayn's way to Kufa with a retinue of about 70 men, his caravan was intercepted by a 1,000-strong army of the caliph at some distance from Kufa. He was forced to head north and encamp in the plain of Karbala on 2 October, where a larger Umayyad army of 4,000 arrived soon afterwards. Negotiations failed after the Umayyad governor Ubayd Allah ibn Ziyad refused Husayn safe passage without submitting to his authority, a condition declined by Husayn. Battle ensued on 10 October during which Husayn was killed along with most of his relatives and companions, while his surviving family members were taken prisoner. The battle was the start of the Second Fitna, during which the Iraqis organized two separate campaigns to avenge the death of Husayn; the first one by the Tawwabin and the other one by Mukhtar al-Thaqafi and his supporters.

The Battle of Karbala galvanized the development of the pro-Alid party (Shi'at Ali) into a distinct religious sect with its own rituals and collective memory. It has a central place in Shi'a history, tradition, and theology, and has frequently been recounted in Shi'a literature. For the Shi'a, Husayn's suffering and death became a symbol of sacrifice in the struggle for right against wrong, and for justice and truth against injustice and falsehood. It also provides the members of the Shi'a faith with a catalog of heroic norms. The battle is commemorated during an annual ten-day period during the Islamic month of Muharram by Shi'a, culminating on tenth day of the month, known as the Day of Ashura. On this day, Shi'a Muslims mourn, hold public processions, organize religious gathering, beat their chests and in some cases self-flagellate. Sunni Muslims likewise regard the incident as a historical tragedy; Husayn and his companions are widely regarded as martyrs by both Sunni and Shi'a Muslims.

After the third caliph Uthman's assassination by rebels in 656, the rebels and the townspeople of Medina declared Ali, a cousin and son-in-law of the Islamic prophet Muhammad, caliph. Some of Muhammad's companions including Talha ibn Ubayd Allah, Zubayr ibn al-Awwam and Mu'awiya ibn Abi Sufyan (then governor of Syria), and Muhammad's widow A'isha, refused to recognize Ali. They called for revenge against Uthman's killers and the election of a new caliph through shura (consultation). These events precipitated the First Fitna (First Muslim Civil War). When Ali was assassinated by Abd-al-Rahman ibn Muljam, a Kharijite, in 661, his eldest son Hasan succeeded him but soon signed a peace treaty with Mu'awiya to avoid further bloodshed. In the treaty, Hasan was to hand over power to Mu'awiya on the condition that Mu'awiya be a just ruler and that he would not establish a dynasty. After the death of Hasan in 670, his younger brother Husayn became the head of the Banu Hashim clan to which the Islamic prophet Muhammad also belonged. Though his father's supporters in Kufa gave him their allegiance, he would abide to the peace treaty between Hasan and Mu'awiya as long as the latter was alive.

The Battle of Karbala occurred within the crisis resulting from the succession of Yazid I. In 676, Mu'awiya nominated his son Yazid as successor, a move labelled by the historian Wilferd Madelung as breach of the Hasan–Muawiya treaty. With no precedence in Islamic history, hereditary succession aroused opposition from several quarters. Mu'awiya summoned a shura, or consultative assembly, in Damascus and persuaded representatives from many provinces to agree to his plan by diplomacy and bribes. He then ordered Marwan ibn al-Hakam, then the governor of Medina, where Husayn and several other influential Muslims resided, to announce the decision. Marwan faced resistance to this announcement, especially from Husayn, Abd Allah ibn al-Zubayr, Abd Allah ibn Umar and Abd al-Rahman ibn Abi Bakr, the sons of Muhammad's prominent companions, all of whom, by virtue of their descent, could also lay claim to the caliphal title. Mu'awiya went to Medina and pressed the four dissenters to accede. He followed and threatened some of them with death, but they still refused to support him. Nonetheless, Mu'awiya convinced the people of Mecca that the four had pledged their allegiance, and received allegiance from them for Yazid. On his return to Damascus, he secured allegiance from the people of Medina as well. There was no further overt protest against the plan for Yazid's succession. According to the historians Fitzpatrick and Walker, Yazid's succession, which was considered as an "anomaly in Islamic history", transformed the government from a "consultative" form to a monarchy. Before his death in April 680, Mu'awiya cautioned Yazid that Husayn and Ibn al-Zubayr might challenge his rule and instructed him to defeat them if they did. Yazid was further advised to treat Husayn with caution and not to spill his blood, since he was the grandson of Muhammad.

On his succession, Yazid charged the governor of Medina, al-Walid ibn Utba ibn Abu Sufyan, to secure allegiance from Husayn, Ibn al-Zubayr and Abd Allah ibn Umar, with force if necessary. Walid sought the advice of his Umayyad relative Marwan ibn al-Hakam, who suggested that Ibn al-Zubayr and Husayn should be forced to pledge allegiance as they were dangerous, while Ibn Umar should be left alone since he posed no threat. Walid summoned the two, but Ibn al-Zubayr escaped to Mecca. Husayn answered the summons but declined to pledge allegiance in the secretive environment of the meeting, suggesting it should be done in public. Marwan told Walid to imprison or behead him, but due to Husayn's kinship with Muhammad, Walid was unwilling to take any action against him. A few days later, Husayn left for Mecca without acknowledging Yazid. He arrived in Mecca at the beginning of May 680, and stayed there until the beginning of September.

Husayn had considerable support in Kufa, which had been the caliphal capital during the reigns of his father and brother. The Kufans had fought the Umayyads and their Syrian allies during the First Fitna, the five-year civil war which had established the Umayyad Caliphate. They were dissatisfied with Hasan's abdication and strongly resented Umayyad rule. While in Mecca, Husayn received letters from pro-Alids in Kufa informing him that they were tired of the Umayyad rule, which they considered to be oppressive, and that they had no rightful leader. They asked him to lead them in revolt against Yazid, promising to remove the Umayyad governor if Husayn would consent to aid them. Husayn wrote back affirmatively that a rightful leader is the one who acts according to the Qur'an and promised to lead them with the right guidance. Then he sent his cousin Muslim ibn Aqil to assess the situation in Kufa. Ibn Aqil attracted widespread support and informed Husayn of the situation, suggesting that he join them there. Yazid removed Nu'man ibn Bashir al-Ansari as governor of Kufa due to his inaction, and installed Ubayd Allah ibn Ziyad, then governor of Basra, in his place. As a result of Ibn Ziyad's suppression and political maneuvering, Ibn Aqil's following began to dissipate and he was forced to declare the revolt prematurely. It was defeated and Ibn Aqil was killed. Husayn had also sent a messenger to Basra, another garrison town in Iraq, but the messenger could not attract any following and was quickly apprehended and executed.

Husayn was unaware of the change of political circumstances in Kufa and decided to depart. Abd Allah ibn Abbas and Abd Allah ibn al-Zubayr advised him not to move to Iraq, or if he was determined, not to take women and children with him. The sincerity of Ibn al-Zubayr's advice has been doubted by many historians, however, as he had his own plans for leadership and was supposedly happy to be rid of Husayn. Nevertheless, he offered Husayn support if he would stay in Mecca and lead the opposition to Yazid from there. Husayn refused this, citing his abhorrence of bloodshed in the sanctuary, and decided to go ahead with his plan.