I noticed that the diplomacy system can be abused quite easily.
If you wish to fight another realm, you can simply join in without declaring war against anyone.
You could defend your allies land by signing an alliance, fight your enemy anywhere even loot enemy regions rogue, without declaring war.
Now there is nothing wrong with that in my opinion per ce.
But it does help you circumvent diplomacy game mechanics, where your peasants would protest against wars they don't like(which is a quite powerful feature).
As long as relations are on neutral, aggressive or murderous settings will be enough to engage the enemy alone, thus allow you do anything else you'd be able to WITH a war declaration then without.
I've seen that its definitively worth it to circumvent these mechanics as they can cause allot of trouble.
As others have said, this is known and expected behavior, with quite a few caveats and gotchas.
To put it plainly, an Alliance will allow you to
help someone defend. If your ally is the defender, you will join in to help defend them against attackers. However, simply being allied/federated with someone will NOT cause your army to join an attack with them against another realm. If you want to join in the attack, then you have to be at war with the defender. If you try and just set Murderous setting to get around that, then you may very well end up being the Attacker against your ally who is the Defender.
In addition, your enemies can use these same loopholes (they're not really abuses)
against you. If they can arrange to be the defenders, then they can prevent the not-at-war-enemy from joining the battle. They can steal the walls right out from under you by having neutral allies move in and camp there for a turn change, then following them in a turn later.
The addition of other realms to the mix, who also have confused diplomacy, can even result in some realms switching sides. We had a battle on AT with Suville where, due to AT's totally F'ed up diplomacy, Tara ended up attacking the people they were defending-but-not-allied-with, because they are allied-but-at-war-with Strombran who is allied with Suville who is at war with Caergoth ... Damn, I can't even remember the whole situation, it's so screwed up. BUt I sure remember laughing my ass off seeing those Tarzans attacking the people they are supposed to be defending.
So, yeah, you
can use the "loopholes" in the diplomacy system to wage a war against other realms without declaring the war. But unless you can describe in detail the seven steps that the battle system uses to set up the sides in a battle, don't expect it to work like you think it should. And when it blows up in your face, don't come to the forums complaining about how the system is full of bugs. Or, go ahead, and we'll explain how it worked exactly as designed, and then get a good chuckle out of how you and your "enemy" teamed up to massacre the people you were trying to defend because a group of four monsters wandered onto the field where your three 20K CS armies were about to wage a glorious battle,
