When playing as a pacifist, I sometimes got wars declared on me, but I wasn't willing to commit to these wars. In these times I just put humiliation into my list of demands, send some forces to blockade their planets and peace right back out.
I don't do this all to often, because I could be liberating instead, but sometimes you are caught while still recovering from another war, mostly in early game. Once you are powerful enough, the AI will be much more careful about declaring war.
However, if you know you can crush the opposing empire, just take what you can.
For instance, I was fighting a liberation war against a small empire, when another declared war on me. The little aggressor on the other side of my empire just ran out of colonizable worlds and thought its two planets worth of fleet and economy could tip the balance. Humiliated them with a reinforcement fleet.
Then ten years later, they experienced a change in government and became a vassal.
My rule of thumb: If you want their stuff and are powerful enough, knock them out. If you realize you bit off more than you can chew, making peace is still better than losing to the AI's war demands.
Another thing about the AI. It seems more likely, that it accepts your initial war demands and I didn't see the AI change its own demands either.
Personally, I think that the game needs to have more options of war and diplomacy.
Mercenaries, for instance, could be Fleets I dispatch to fight in my allies war, but under their control, lining my pockets with extra minerals, energy and influence.
Same thing with licensed production. I want to be able to rent out my starports production capacity to a friendly empire. I decide the prices, so theoretically I could be running at a loss, but most likely will have some extra income for my empire, because most of my starports are idle during peacetime.
I want to give lend-lease agreements to other empires in return for favours, research agreements, or even uneasy alliances.
Naturally, this behaviour is dubious enough to rustle the jimmies of my pacifist citizens, but since I am technically at peace, they shouldn't be too mad about it.
At the same time allowing you to pay other empires to construct your ships may allow you to win a tough war, but you have to buy their designs and refit later.
This will allow groups of smaller empires and even larger empires (especially in multiplayer) to have meaningful cooperation and diplomacy.
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