Well, you can totally annihilate new recruits. Generally you only get 1% war score. I suppose that is because the number of new recruits is usually few.
Okay, you also get or lose more than 1% war score if there are armies from multiple countries present. You can get 1% (or 3%) war score for each country defeated. But that is not what I'm talking about.
It is not based on number of troops killed, unless that is another case where you can get 3% war score. Similarly, it is not for defeating just any army larger than yours unless that is another special case. Let me tell you what happened in my current Venice game.
I was at war with Sweden, and I had a stack of over 50K troops in the capital. Sweden had a stack of over 50K troops in a neighboring province. I had military access and a blockade, so I had over 100K supply in Sweden's capital. However when I cycled my ships to port, I temporarily had only a little more than 50K supply. The AI saw this as an opportunity to strike and cause my army maximum attrition. The AI advanced, but my ships were back in place in no time.
However the AI proceeded with its plan to cause attrition to my army. When the AI came into contact with my army they just fled after one day. Oddly enough, I had enough supply, but Sweden did not. I kept reducing Sweden's army through attrition, and I got 3% war score for each battle. There were no significant casualties from battle because each battle lasted only one day. Due to reinforcements, Sweden's army was never smaller than 40K. I always had the larger army.
