^ You don't need oil for cavs. You do with tanks however.
Anyway, how did you get that far in your first game. I didn't even get to the industrial age in my first 5 or 6 games.
Leaving 2 full armies of cavalry in a newly conquered city, just so it can flip, and take my armies... Or do they just die? I don't even know how it works. But I lost my two armies!
Forgetting to always treat the AI as an enemy to be destroyed.