Hill's inspiration, as he told The Chronicle last year on the occasion of a retrospective of his work at UC Berkeley's Pacific Film Archive, "came from Greek history: the story of Xenophon and the march of 10,000 (mercenary soldiers) to the Black Sea."
Interesting that he doesn't mention 'The Warriors' by Sol Yurik, which is the book the movie is based on. Yurik wrote it based on the Xenophon story.
Also, in the book the main character is black, but Hill used a white guy-Michael Beck.
They're not as nice in the book either, there's much more violence and a rape as well
But it is one of those rare instances where the movie is better than the book