It's just a matter of giving each faction enough victories and be able to downsize the defeats. Alliance takes a zone, Horde takes another. You can quest through a losing battle, and yet your actions change the outcome enough to fill they mattered. Both sides could lose heroes. The war could end with each side gaining one main objective, but neither being able to overcome the other.
The thing with war in WoW so far is that it doesn't feel like a war most of the time. You don't really participate in a war campaign.