When God Broke His Vow

Before the beginning of the battle of Mahabharat, Lord Krishna took a vow that he will not lift a weapon during the battle. This vow was well known to everyone.
Bhishma Pitamah was the commander in chief of the Kaurava army and was a God-realized saint. He also knew about His Lord's vow. Still, saints act in strange ways. With a loving bhav, Bhishma Pitamah pledged to to make Shri Krishna pick up a weapon again him.
On the fourth day of the epic battle of Mahabharat, Bhishma Pitamah decided to arrange his army in an Eagle like formation. Pandavas chose to arrange their army in a semi-lunar formation to counter the offensive of the Kauravas. When the battle began in the morning, the second oldest Pandava prince, the mighty Bhima attacked the eldest Kaurav, Duryodhana with powerful arrows. Duryodhana fell unconscious under the impact of Bhima's arrows and had to be moved to a safe place.
Duryodhana was not happy with the way his grand-sire, Bhishma was commanding the army. He thought that even though Bhishma was leading the Kaurava army, he still supported the Pandavas. He kept insulting Bhishma time and again. Bhishma decided to step down as the commander in chief of the Kaurava army, but was persuaded by Guru Dronacharya other senior Kaurav leaders to continue to lead the army.
As the battle progressed during the day, Bhishma attacked Pandavas with relentless energy. Pandava army was badly hit. Hundreds and thousands of soldiers were killed on the battle field. Sri Krishna instructed Arjuna to attack Bhishma to protect the Pandava army. He told Arjun that there was no other way for Pandavas to win the battle, other than killing Bhishma. A terrible battle ensued between the two great warriors - Arjun and Bhishma.
Bhishma's arrows pierced Arjuna's body. Arjuna started bleeding profusely. Seeing this onslaught of arrows from Bhishma, Krishna became furious. Clinching His teeth in violent anger, Shri Krishna picked up a chariot wheel and rushed towards Bhishma.
Seeing the Lord Himself ready to attack, Bhishma laid down his weapons. With tears flowing from his eyes Bhishma spoke, "O Keshava, O Merciful, how blessed am I that the Lord of Universe Himself has decided to liberate this ordinary Bhishma from the bondage of worldliness. O Madhava, please let me die at Your hands. There can be no-one luckier than me to receive such a wonderful and rare opportunity".
Bhishma's vow to make Shri Krishna pick up arms against him thus became true. Sri Krishna broke his vow of not picking up arms in the battle of Mahabharat. All for the happiness of his ardent devotee-Bhishma Pitamah.
These words of Bhishma Pitamah reminded Sri Krishna of His vow. He threw away the wheel and retreated back to His chariot.
Soon after, as the evening approached and the sun started setting, the battle ended for the day and the soldiers returned to their camp.
Divine Law: God always fulfills the wishes of His devotees, even if it is unreasonable. Such is His love for His devotees