Excerpts from Books and Wikipedia
The Bhagavad-Gita (Song of the Lord) is part of the Mahabharata; it was written in about 400 BC to 100 BC. Krishna, a central character in the story, is a manifestation of Vishnu. [Great Courses: Hinduism] "This greatest single event in India's ancient history [Bharata war], and the inspiration for the world's longest poem [Mahabharata], did not occur until 'c1400 BC' according to the History and Culture of the Indian People . . ." [Keay: India: A History, p. 3]