It's also worth noting that being the aggressor does carry a bit of a penalty to it - I know you didn't start the war, but making the attack move against enemy troops even on your own territory is probably considered bad by the pope. I'm not sure this is the case, but it may be, especially when you're at the same or lower papal approval level as your opponent. You're probably far safer parking the army in the town of that province and waiting for the enemy army to come try to take the town. Of course as others have pointed out, if you have a higher papal rating than your opponent, you typically have a lot of leeway from the papacy, enough often to even support land-grabbing forays into enemy territory. Another big thing is that having a good papal advantage typically makes your opponent get the warnings from the pope while you're at war. They often fail to heed them, and as a result will often plummet in rating even further, usually all the way to excommunication, at which point you can do anything you bloody well want to them, free of (papal) charge. So it's not only good for more leeway as an attacker, it also punishes your enemies more too, which makes them even easier for you to lean on. Papal standing is awesome, and the price is always worth paying