What was the reason for the fighting between Ali and Mu'awiyah—may Allah be pleased with them both—in the battles of Al-Jamal and Siffin, and did they forget the sanctity of a Muslim's blood?

The belief of Ahl al-Sunnah wa al-Jama'ah is to refrain from discussing what transpired between the Companions, to be pleased with them, and to believe that they were mujtahids (exercised independent judgment). Fabricated reports that diminish their standing should not be relied upon. The Battle of Siffin occurred between the army of Ali and Mu'awiyah, may Allah be pleased with them both, due to a disagreement over the killers of Uthman. Mu'awiyah demanded retribution, while Ali demanded allegiance first. The Companions did not forget the sanctity of Muslim blood, but what happened was Allah's decree. The Companions unanimously agreed that both parties were believers. The Qur'an called the warring factions believers and made them brothers, and the Companions set the finest examples of brotherhood and interaction between warring parties. The Battle of the Camel was not between Ali and Mu'awiyah, but between Ali and Talhah, Al-Zubayr, and the Mother of the Believers, Aisha.