In your example, I would have to say you must go to win, since you need the water to survive, which implies that there would be resistance from the beings currently living there. It appears that you would either have to exterminate or enslave those who would resist, or make some kind of agreement with them that you would not take every drop of their water all at one time, which would doom their planet.
However, using earth as an example, our planet is 70% covered by water, but only one percent is drinkable. You used the term "fresh water" in your post, but would the leaders viewing the cameras know if the water on another planet might be poisonous, or otherwise unfit to drink before the war started to get that water? It would be poor planning to go to war over a resource that proved worthless to the invaders.
Off topic, but how is Ce'Nedra faring these days?