The Bhagavad Gita is a holy book in the tradition of Hinduism, and it is a story of Yoga. In this story, the main character is Arjuna, and it is set in the midst of a war. It almost seems contradicting that a book about spirituality is a book set in war, and the dilemma for Arjuna is that he is hesitant to fight in the battle. He is conflicted because he feels that fighting is violent and is a contradiction to the principle of ahimsa. His God, Krshna, tells him that if he does not fight this war, more damage will be done, more casualties will occur, and that it is his role to fight in this war.
The reality is that in this life, there will be many battlefields. Ahimsa or nonviolence is not an impossible perfection. It is choosing to minimize the harm that we cause in the world. While acknowledging that even as we may not be able to stop all of the wars in all the world singlehandedly, we can choose to act instead of turning away from it all. We can choose always to do something over nothing. Furthermore, nonviolence is not inaction. Nonviolence is the conscious active decision to participate.
All spiritual teachings teach nonviolence. Religions may have different ways of expressing them, as Christianity has the 10 commandments, Buddhism has the 8-fold path, and Yoga has the 5 yamas. But these are not just sets of rules that one must follow because we are told. Rather, they become a natural progression of how we live in this world when we are truly connected with the deepest intentions of our heart-- to stop war, to have peace, to let go of separation, and to be connected to the intricate web of life that we are a part of.