Bhagavad Gita is a deep scripture about yogic teachings.
The premise of the story is a war between a family fighting for dominance.
Arjuna is one of the greatest archer at the time and he was fighting against one side of his family.
Both side sought out Lord Krishna for help for their cause. Lord Krishna gave each a choice - His army or Him as a guide.
Arjuna chose Him as a guide and majority of the book is about the dialog between them.
The scripture is deeply allegorical and metaphorical. Bayasa, the author, wrote it as a guide to live one's life (Self-Realization).
The war is actually talking about the "war" every human being is fighting inside themselves. The "good" versus the "bad" or virtues versus vices. That's the whole reason why we suffer. It's all an inner journey :)
I'd highly recommend reading The Autobiography of a Yogi by Paramhansa Yogananda as his Master and other great saints explain more about it than I can.