Quote Originally Posted by MarcusAureliusAntoninus
If you win, that proves that your military is superior and doesn't need to be changed. You won without adopting their style of warfare, why would you adopt an obviously inferior military system.

Though, looking through some stuff, I'm not sure if you actually have to lose. I think it is actually something like: have several different generals fight against cataphracts...
No, losing the battle is essential. It has to be a big loss too, with high casualties not only on your army as a whole but with the general's bodyguards as well. On the other hand, you don't have to lose to Parthians specifically, you can lose to Baktrian or Armenian armies as well.