I've had problems getting peace treaties when the cause of war was a campaign objective region. For example, with Prussia, after taking Rhineland and Flanders (owned by Westphalia at the time), the French, who were very friendly with me, declared war. I wiped out their stacks in France and besieged Paris. I made a peace proposal, but it was rejected. I tried again, this time with a few tech thrown in, still rejected. My last offer, 100k in cash, Flanders region (the reason why they went to war in the first place), and the techs I threw in earlier. Still, rejected emphatically. Oh well, captured Paris and didn't bother to stick around, the following revolution bought me about 10 turns of peace and quiet.