Its the Rome: Total War demo. I'm downloading it right now. On the history channel show that uses the same engine, they were talking about sparticus or somebody like that, and how he had an army that was 100,000 strong, I doubt the game could do that many people. It never actually showed all the troops at once, so they probably filmed it in parts.
"As we've mentioned before you'll be downloading the tutorial for Rome: Total War as well as the Battle of Trebia, one of the historical battles included with the final product. Along with that is a flash file with more great screens and information for your digestion."
That's what IGN says the demo is.