Why did Bhishma, such a great devotee of Lord Krishna, oppose Krishna and Arjuna in the Battle of Kurukshetra?

by Laxmimoni dasi

Bhishma is a Mahajana (one of Krishna's greatest devotees), and he achieved spiritual perfection in his relationship with Krishna by serving the Lord in a chivalrous mood. He was fully aware that Krishna's will would be done no matter what side he himself fought on, so he chose to do his duty and fight with those for whom he was working (the Kuru family).

Another consideration was that Bhishma had taken a vow, when he refused to marry, that he would remain and protect the ruler of Hastinapur (capital city of the Kurus) until the kingdom was safe. And although he had a benediction that he could die at will, he chose to remain alive until he could see that the Pandavas were in their rightful place. His participation in the battle was with his body, but his heart and blessings were always with the Pandavas.