Excerpts from Books and Wikipedia
"Krishna, the Manifestation of God, brought a new civilization. He delivered man from evil and sorrow. He assured His followers that in the future God would again manifest Himself to repeat what Krishna had done. To guide the wandering people of the world to the straight path of God, He said: 'Whenever there is a decline of righteousness and rise of unrighteousness, O Bharta (Arjuna); then I send forth Myself for the protection of good, for the destruction of the wicked and for the establishment of righteousness. I come into being from age to age.'” [Fatheazam: New Garden, p. 24]
The poem about Him existed first as an oral tradition; Written composition reached a mature form probably in about the 4th century BC; "Therefore, arise, thou Son of Kunti! Brace Thine arm for conflict, nerve thy heart to meet- As things alike to thee- pleasure or pain, Profit or ruin, victory or defeat: So minded, gird thee to the fight, for so Thou shalt not sin!" Bhagavad Gita (Edwin Arnold tr.)