Take a look at this thread:
https://us.forum.elvenar.com/index.php?threads/combat-101.4968/
I didn't even train troops for first 3 chapters; was a 'cater only' city. Then I reassessed that decision and had to start learning (and furiously training, lol!). This is the thread I used to start learning, then I went into the more detailed battle guides.
There is also a lot of good information and videos at:
https://www.gamersgemsofknowledge.com/
The hardest chapters for fighting are the early ones, as you don't even get all the troop types until later. Also, understanding how the difficulty of fighting increases is crucial as well. The provinces are rings around your city. Each ring has a similar difficulty level. So, if you don't scout in a circular pattern (ex: it's more of a spider pattern as you stretch to reach undiscovered neighbors or to do specific types of provinces for the relics), you'll have more variation in the difficulty rating of battles. There is a hard stop at ring 11, where you will not be able to win fights and you won't have orcs (not the troops, the resource) to cater. Orcs are a resource like goods, but are made by your armories. However, your armories don't get the ability to make them until you upgrade them to lvl 20. You'll get the tech in the research tree to be able to do this early in Chapter 8. If you get to ring 11 before you get to that research, you'll be stuck until you can get there. There's not much in-game to let you know this is the way the game works. Also, early on you need lots of expansions and it seems the easiest way to get them is to explore more provinces. This will also cause pain in the long run, because your battles just get harder and harder the further you are away from your city. Activating the Advanced Scout tech for the next chapter reduces the difficulty of un-scouted provinces (doesn't change the ones already scouted, but not yet completed). The chest for the next chapter shows the minimum provinces to be able to proceed to the next chapter. You can exceed this amount by a little; I'm not sure I've ever seen specific numbers on this. The amount you can go beyond the minimum increases the further along in the game you are, but this is mostly a function of the rings getting larger as you move along, thus having more provinces per ring. It's a lot of information to process. I suggest starting in the Combat 101 thread, trying it out in tournaments (where you'll have smaller squad sizes and thus lose fewer troops) watching some of the videos on Gems of Knowledge, and learn the battle pentagon. You can find the pentagon in Mykan's Fighting Guide, a sticky in the battle help forums. There's lots more, and I'm sure others will be along to add things, but this should give you a starting place.