You fight with what you have but then sometimes you lose to someone who has a better army than you do and disappear from history. So in the long run you end up

You fight with what you have but then sometimes you lose to someone who has a better army than you do and disappear from history. So in the long run you end up fighting with what actually works in your terrain. And I think it's very interesting that horses, who are only really indigenous to the steppe, ended up proliferating over the entire earth in a way that elephants did not. Hundreds of elephants on the battlefield sounds great if you can feed them though!