Excerpt from Yoga, Ahimsa and Terror by David Frawley:
The Bhagavad-Gita, which teaches about the spiritual aspect of yoga in great detail, was taught on the battlefield, during a civil war. While some will say that this outer battlefield is a metaphor for an inner struggle, which is true, that an outer battle was involved is clear from many historical records from ancient India. Krishna, the great yoga teacher, encouraged his disciple Arjuna, who was a great warrior, to fight, though Arjuna was reluctant and wanted to follow a way of non-violence instead. Why did Krishna encourage Arjuna to fight?
There are two main types of ahimsa in the Yoga tradition. The first is ahimsa as a spiritual principle, that followed by monks, yogis and sadhus, which involves non-violence on all levels. The second is ahimsa as a political principle, the ahimsa of the warrior or the Kshatriya, that is followed by those who govern and protect society, which allows the use of violence to counter evil forces in the world, including to protect spiritual people, who often cannot defend themselves and become easy targets for worldly people. Krishna taught this Kshatriya ahimsa to Arjuna for the benefit of future generations. Sages before Krishna also taught this, like Vishvamitra who taught Rama and Lakshmana to destroy the evil forces that were persecuting spiritual people, so it is a very old tradition of India.
Yoga teaches us about the three great qualities of nature, the gunas of Prakriti, of sattva (harmony), rajas (action and aggression), and tamas (inertia, ignorance). There are several important laws of the interrelationship of these gunas. One important law is that sattva cannot defeat tamas. The quality of sattva being harmony, balance, meekness and surrender cannot break up the inertia of tamas, which is deep-seated anger rooted in ignorance, hatred and violence. For this the application of rajas or action to force change is required. Sattva or harmony cannot survive unless rajas is used to suppress tamas, which sees sattva as an unarmed enemy.