How did the discord begin during the era of the Rightly Guided Caliphs, and what was its cause?

The first tribulations after the passing of the Prophet, peace and blessings be upon him, were the apostasy of the Bedouins and the refusal of some groups among them to pay zakat. Abu Bakr al-Siddiq fought them until they returned to Islam. Then came the tribulation of the killing of Uthman, may Allah be pleased with him, which led to the division of the Ummah. This was followed by tribulations during the era of Ali, may Allah be pleased with him, such as the Battle of the Camel, Siffin, and Nahrawan. The أهل السنة والجماعة (Ahl al-Sunnah wa al-Jama'ah) believe that the wars among the Companions were not due to whims, but rather a pursuit of the truth, and each of them is rewarded, whether he was a mujtahid (one who strives to deduce legal rulings) who was correct, in which case he receives two rewards, or one who erred, in which case he receives one reward for his ijtihad.