I think no one answered you because the combat system is so daunting no one wants to tackle explaining it.
I'll tell you this though: it's very solid.
There are some rough edges as far as some things go in the combat system (mainly dealing with aircraft), but battles still play out well and predictably, with organization and tech levels playing a big factor.
And luckily there isn't early game EU2 syndrome where your 30k army would lose to a 5k one when you had twice the tech level and more morale.

Just don't play Poland.
