Mahabharatha is a great and bloodshed war fought on Kurukshetra by the mighty armies of Pandavas and Kauravas. It was foutght for 18 days. Krishna gave his army to fight on the kauravas side. He took a vow that he would not fight in the battle but agreed to help and save the pandavas and their army. During the war he became the Charioteer of Arjuna. From the first day onwards Bhishma was matchless and showed his prowess. Even Arjuna, the great archer could not contain Bhishma.
Bhishma is a great devotee of Krishna, he knows very well that Krishna himself is the supreme lord Srimaha Vishnu. Bhishma wanted to see the mighty power and valor of him and wanted to die in the hands of him. So, he fought ferociously with Arjuna to provoke Krishna to fight him. By this Krishna was furious and broke his vow. He alighted the chariot and took a chariot wheel and aimed at Bhishma. Looking at this, Bhishma felt very happy that he would be killed by Krishna. Then Arjuna jumped off the chariot and stopped him, he fell on Krishna’s feet and pleaded him not to kill Bhishma and break his vow. Within a second Krishna would have killed Bhishma if Arjuna did not stop him.
This incident proves that god himself breaks his vows to the wishes of his devotees. God needs nothing but belief on him and god, in return shows grace and protects his devotees forever.