What is the Battle of al-Harrah, and who was the first conqueror of Constantinople?

The incident of Al-Harrah was a tribulation (fitna) that occurred between the people of Medina and the army of Yazid ibn Mu'awiyah in the year 63 AH. Its cause was the people of Medina's renunciation of their oath of allegiance to Yazid due to reports of his فساد (corruption/immorality). Some companions, such as Ibn 'Umar, opposed the renunciation of allegiance to Yazid, and mentioned that the Prophet, peace be upon him, said: "Whoever withdraws his hand from obedience, will meet Allah on the Day of Judgment with no argument for himself. And whoever dies without an oath of allegiance around his neck, dies a death of ignorance (Jahiliyyah)."

Yazid sent an army led by Muslim ibn 'Uqbah, who killed and plundered the people of Medina and forced those who remained to pledge allegiance to Yazid. For this, he was nicknamed "Musrif" (the extravagant/one who exceeds limits).

Yazid was the commander of the first army to conquer Constantinople, and the Prophet, peace be upon him, gave glad tidings of forgiveness to this army. This does not necessarily mean forgiveness for every individual in it, as a strong impediment might prevent the realization of the cause.

Ahl al-Sunnah (definition) (Sunnis) believe that Yazid was a Muslim ruler who had good deeds and bad deeds, and was not an unbeliever. He mixed good deeds with bad ones, and his affair is with Allah.