What is defense.

The first 5 if you don't attack with them will be defending the other 10 will do both.

if your reinforcements manage to merge with your original stack the entire group will be defending only.

If your reinforcements do no merge you will see a progress circle above their icon meaning they are attacking once every hour using the attack value of the unit and are getting attacked once an hour using the defence value for the unit.

What makes reinforcements sometimes merge and sometimes not? I have no idea

Kanaris wrote:

What makes reinforcements sometimes merge and sometimes not? I have no idea

Each army group attacks every 1 hour. When your reinforcements arrive, they are unlikely to be on the same schedule, and therefore are on a separate attack/defense timer.

If they are close enough, sometimes the game merges the units.

This is the point when I jump in with SBDE.

SBDE (State-Based Damage Efficiency, which you can read about by clicking here) max value for infantry is 8, meaning that if you have 5 infantry to begin with, and you add 10 more, you are at 15 infantry, which is far over the limit. this means that you will have 7 infantry doing nothing, increasing your losses.

Actually 15 infantry fights like 12 infantry after SBDE is taken into account.

In any case, sometimes you want to reduce your frontage so the battle last longer, for instance if you are artillery as primary damage dealers or reinforcement coming.

Not exactly: they incur damage to the enemy as 12, but they spread the received damage on the 15 units (i.e. they last much longer). From that aspect, it is clear that (overstacking or not) they will last much longer.

Of course, the main defect of overstacking (between comparable armies) is that if you overstack in one area (with your units operating in a less than optimal way), you will be left weak in another (allowing a flanking operation, where your opponent's units will operate in an optimal way).

Yeah the important thing to remember about SBDE is that each troop's stats decline, they don't drop off after a certain point. If you have 8 infantry and another close by, just add it in it doesn't really matter if it's over the SBDE limit. Also check each troop's stats- most are balanced but some are much better at attacking or defending, so if someone is attacking your garrison sometime it is a smart idea to go meet them, or to lure someone to attack your defensive troops like AT guns.