The Kurukshetra war · Mahabharata questions
Why did Krishna not fight in the war?
Krishna offered both sides a choice before the war: his whole army, or himself alone, unarmed and not fighting. Arjuna chose Krishna, Duryodhana took the army. Krishna kept the condition by serving as Arjuna charioteer and counsellor, and twice almost broke it in anger.
Before the fighting, Duryodhana and Arjuna both come to Dwaraka to ask for Krishna help and find him asleep. Duryodhana sits at his head, Arjuna at his feet. When Krishna wakes he sees Arjuna first, and offers a choice which he gives Arjuna the right to make as the younger man: on one side the Narayani sena, his entire army, on the other Krishna himself, who will carry no weapon and will not fight. Arjuna asks for Krishna. Duryodhana, delighted, takes the army, and thinks he has won the negotiation.
The condition is kept in the letter. Krishna drives Arjuna chariot, which is the work of a servant, and the epic makes a point of the god of the world holding the reins and washing the horses wounds in the evenings. His weapons in the war are counsel and timing: he tells Arjuna when to attack, has Ghatotkacha sent in to force Karna to spend his one unfailing spear, tells Yudhishthira how to break Drona, and reminds Arjuna of the rules already broken when Arjuna hesitates over an unarmed Karna.
Twice he nearly breaks the vow, both times against Bhishma, when the grandsire is destroying the Pandava ranks and Arjuna is fighting half-heartedly. Krishna leaps from the chariot and takes up a wheel as a weapon, and Bhishma welcomes death at his hands. Arjuna stops him each time by promising to fight properly.
There is a larger reason inside the theology. Krishna argument in the Bhagavad Gita is that the war is already decided and that Arjuna is the instrument, not the cause. A Krishna who fought would make the outcome his; a Krishna who only drives makes it Arjuna, which is what the Gita is for.