Not sure if you are referring to the ship roles as they are in game right now or not... Capital ships currently prioritize escorts first, then capital ships. (Capital ships will not try to shoot guardians first, see ship roles at http://galciv3.gamepedia.com/Ship_roles

In your Capital/Escort fleet vs my Capital/Guardians fleet:

Your high attack capital ships would target my high defense capital ships. I win there.

Your high defense escort ships would target my high defense capital ships. Defense ships do very little damage.

My high attack guardians would target your high attack Capital ships (that also have low defense) thus tearing them to shreds.

My high defense capital ships target your high defense escorts. Defense ships do very little damage.

The defend part means that they will attack ships that are attacking the ship they are defending. This is why the guardians will target the capital ships that are shooting at its ally capital ship.

You are right... Just did some testing, and what I stated it not how defend works. I am not sure how defend works, but I have a saved game where I can do some testing now.

My fleet of (1 escort and 4 capital ships) went against enemy fleet of (1 assault, 1 interceptor, 1 escort, 1 support). All the enemy ships started battle targeting my 1 escort which I expected. But my escort started battle targeting the interceptor! I cannot explain why, but somehow, defending my capital ship caused it to go after the interceptor first. (I am discussing targeting as displayed on tooltip in game, not who actually gets shot at first due to primary target being out of range.) If I send the 1 escort alone against that same enemy fleet, my escort starts combat targeting the assault ship, not the interceptor. I do not know how targeting works for ships that are defending. They must be using a different targeting priority list when defending.

Yes... I believed that defense worked as you explained at first but experimentation seemed to indicate otherwise. Which is why Escort/Capital seem to be the optimal configuration. With escort just fully defense and capital just pure offense. Although I usually put at least one or a very small amount of weapons on my escort ships, but only the bare minimum.

Very interesting, so if I understand this correctly, I should make missile boats support ships, this will cause them to stay in back and maximize their range.

I also dislike how the Escort/Capital combination can be abused. It feels like the only viable strategy against a difficult enemy, but it comes close to cheating. It could of course be a deliberate design choice, but I don't really believe that.

Like others, I too spent time trying to mod the battlebehaviour, looking for an anti-abuse fix. The only fix that seems to work so-so is to have each role prioritize itself, because then they can't hide behind any another role. But that makes the role name seem silly, since they actually don't mean what they are supposed to mean anymore. And a player can still abuse it, by putting "just guns" ships in a role that the AI never puts anything dangerous in (like guardian). I am beginning to think that there is no satisfactory solution from just fiddling with the battle behaviour.

Instead, I think that the complete freedom of picking roles is the problem. Problem examples: Huge hulls as Interceptors, Tiny hulls as Capital ships, and ships fitted with weapons in the support role. If this freedom was restricted, it would be easier to create a battle behaviour that is hard to abuse, and still have the role names mean what they should mean.

Example of how restrictions can help:

Assume these limitations:

Interceptor (tiny, small)

Guardian (tiny, small, cargo)

Support (any size but no weapons)

Escort (medium, large, huge, the assault drones from carriers)

Assault (medium, large, huge)

Capital (medium, large, huge)

And a few words about the battle behaviour. The ships are basically divided into two different groups. The first group (escort, assault and capital) have big ships. The second group (interceptor, guardian and support) have small ships. And then each group will go for the identical group on the enemy side first. This seems like the intended behaviour based on the role names, it's not something I made up. What it boils down to is that a fleet can loose its support even though they won the battle, and that is a pretty cool way to stop an invasion fleet.

Target priorities (changes: The top three targets their own role first. Interceptors kill support immediately after guardians)

Capital (starts @ 1400) -> Capital, Assault, Escort then Interceptor, Guardian, Support

Assault (starts @ 1500) -> Assault, Escort, Capital then Interceptor, Guardian, Support

Escort (starts @ 1600) -> Escort, Capital, Assault then Interceptor, Guardian, Support

Interceptor (starts @ 1100) -> Guardian, Support, Interceptor then Escort, Assault, Capital

Guardian (starts @ 1100) -> Interceptor, Escort, Assault, Capital, Guardian, Support

This isn't perfect. But there are no role-combinations where a ship will be safe. And Tiny and Small ships will fill a function late in the game too which i think is nice and adds diversity. It should also be possible to make the AI deliberately protect or attack the support role, just by adding small ships to a fleet.

edit: minor tweaks