Lord Krishna had promised before the battle of Kurukshetra that he would not take up arms, but His Majesty rose to strike Bhishma with the wheel of the chariot. Did it break the word of Lord Krishna? If so, what is the rationale behind it?

Lord Krishna took up arms to teach 'Arjuna and Bhishma'.

                     The battle of Kurukshetra was a battle for the rights of the Pandavas, but the innumerable Rathi-Maharathis and the seven Akshauni armies who fought with them had no personal rights or interests but still took part in the crusade believing in Arjuna's bravery and prowess. It is the duty of Arjuna and other Pandavas to protect them. But Arjun was not fulfilling his duty completely.

                     When Arjuna was fascinated to see his relatives on the first day of the war, Lord Krishna bestowed upon him the knowledge of the priceless Bhagavad Gita and freed him from filth. But even if Arjun's guilt was removed in the case of others, Arjun would be fascinated when he saw his grandfather Bhishma. Arjuna's grandfather could not kill Bhishma but if Arjuna had fought with his full strength, it would not have been possible for Bhishma to kill a terrible army every day. Lord Krishna took up arms to convey this.

                     Secondly, he wanted to teach Bhishma. When Bichitravirya died childless, his mother told Satyavati Bhishma to marry Bichitravirya's two wives, Ambika and Ambalika, to protect the dynasty and to protect Hastinapur from the throne. But Bhishma disobeyed his mother-in-law Satyavati because of his promise. As a result, the Kurubansa was destroyed.

                     Because if Bhishma was the king, he would not have allowed any incident to happen in Hastinapur. But he did not ascend the throne, so that "despite knowing the conspiracy to bury the Pandavas in Barnavarta, he could not say anything." To give and, finally, to cause the defeat of the beloved Pandavas on the side of iniquity". All this he had to endure only because of his promise to remain loyal to the throne for life.

                   Lord Krishna took up arms and told him that "a promise is a personal problem, and if that personal problem becomes the cause of iniquity, then it is better to break that promise."