Why did Yazid assume power after the death of Muawiyah, may Allah be pleased with him? Was that not a clear breach of what Al-Hasan and Muawiyah had agreed upon: that the Caliph be chosen by Shura?

Mu'awiyah ibn Abi Sufyan was a venerable Companion and a scribe of divine revelation. He was known for his forbearance and knowledge, and it is not permissible to disparage him. After the passing of Ali (may Allah be pleased with him), people pledged allegiance to Hasan ibn Ali, who then relinquished the caliphate to Mu'awiyah in the Year of Unity (40 AH) to prevent bloodshed among Muslims. The Prophet (peace be upon him) praised him for this action. Hasan stipulated to Mu'awiyah that he should not appoint anyone as his successor and that the matter of leadership should remain by consultation (shura). However, Mu'awiyah saw the benefit in unifying the word and preventing discord, so he pledged the caliphate to his son Yazid after consulting with senior Companions, many of whom agreed to it, including Ibn Umar. This independent judgment (ijtihad) by Mu'awiyah was to unite the Ummah and bring its ranks together. If he was correct, he would receive two rewards, and if he erred, he would receive one reward.