Why does the Bhagavad Gita begin on a battlefield?

Because it is a dialogue about duty and action, set at the onset of a war. The Bhagavad Gita is a Hindu scripture, likely composed in the second or first century BCE, that forms part of the epic Mahabharata. It is set as a dialogue between the Pandava prince Arjuna and his charioteer Krishna, an avatar of Vishnu, at the onset of the Kurukshetra War.

Arjuna asks Krishna to drive to the centre of the battlefield, so that he can see both armies. He sees that some of his enemies are his relatives, beloved friends and revered teachers. He hesitates. Krishna replies that there is no way to avoid action, since abstention from work is also an action, and he persuades Arjuna to fight, arguing that while following one's dharma one should not consider oneself the agent of action, but attribute all actions to God.

The Gita is a synthesis of strands of Indian religious thought: Vedic dharma (duty), Sankhya-based yoga and knowledge, and bhakti (devotion).

What the battlefield means has been debated. Several modern Indian writers have read it as an allegory for "the war within". Mahatma Gandhi memorised the last 19 verses of the second chapter and considered them his companion in his non-violent movement. Yet Lala Lajpat Rai and Bal Gangadhar Tilak read the same text as defending war when necessary and used it to promote armed rebellion against colonial rule. Interpretations of the Gita have been used to support everything from pacifism to aggressive nationalism.

The framing epic is vast: the Mahabharata is about 1.8 million words, roughly ten times the length of the Iliad and the Odyssey combined.