Some opinions for the ones who want to create a war.
I've to say that I love wars (only in game and fiction obviously) because they are complex and they sustain conflict (the base of every good story).
BUT, as I said, it is really complex. And if you do not take care of every details a war could really get out of hand.
Here some ideas.
First, I agree with Illysia:
Good vs Bad is an old fantasy cliché, there are many people that are tired of it.Yeah, I'm the bad guy, I want to kill you all.
Uhm, why bad guy?
Because I'm EVIL!
Ohh.
I prefer to be a little more creative.
A real war is pretty tied to the setting and to the elements of the society, if you do not look to them the war could be, not natural. Some history examples.
Flower's war (middle america - Aztechs).It started because of famine, then it became a ritual war of the Aztech.
http://en.wikipedia.org/wiki/Flower_warI do not like very much this type of war because it was full of rules, and the soldiers respected these rules...Example, they always fight during daytime.
Damn... I think that wars are a place where people fight with all resources, like surprise attack and all the rest.
Medieval wars (europe).A little more interesting. But short ranged. Too power in few hands. Many times it was personal interest of some noble.
Reinassance (europe).The best for me (for a fantasy setting), many more interest are on board. Guild interest in creating monopoly of some resources, bankers who want rulers to spend their gold, city who want freedom from the rulers, rich families who fight each other, poor people who start rebellions. And much more. Anyone have to take in mind all of these interest. And these wars usually developped on multiple levels. Economic, sieges, open air battles, court intrigues, assassination, also the public opinion started to matters.
Someone used the example of the WWII, well, I think is a little too far away for a fantasy setting. Nationalism, mass media and industrial revolution took a too big part in the world wars. I think that they're not expendable examples for a fantasy war.
Elements that create wars (Be careful, there's a difference between real causes and formal pretext):
Food and famine.
Resources.
Successions.
Trade routes.
Expansion.
Indipendence.
Religion and way of thinking (but is more a pretext or an instrument than real causes).
And remember to answer the question: why a key characters of the plotline want the element? (Ex. why the king want to expand? Or why the banker want so much gold? To show himself? To save a lady in distress? To give a legacy to all of his fifteen sons? For Vengeance?)
To be really accurate, also every player could need to create a motivation for his character if he want to partecipate. But well, that is not always necessary. The important thing is that the key characters (players and/or NPC) have a believable motivation.
Often the wars born from some kind of improvise scarcity.