Narrative & Character motivations

I'm mulling over the two foe's I've concocted, trying to figure out who they are, and why they are fighting in the first place.

The Warcraft 3 clip shows a battle between a human and orc, in a fantasy world with- nope. That's been done to death. BORING!!

Sci-Fi is definately my favorite genre of anything, so that's a good starting point. Two people from two worlds fighting for land? Resources? Women? I think belief is a good place to start. Belief is the cause of so much conflict in the world today, why not echo this with these two foes? Cool. But then, what would drive these two bitter enemies to put aside their differences and stand as one? Survival! I'd sure rather grudgingly accept the help of a enemy than face utter annihilation!

So, what is the conflicting belief in question here? Is it differing religions, or scientific philosophies?

I play Warhammer 40k, which is set in the far future in which every race in the galaxy is embroiled in conflict, each for a bunch of reasons. Of all these races, the two who stand out for me are the Necrons and Eldar. The Necrons are a race of immortal machines who forsook mortal lives to exist forever, where the Eldar are essentially future Elves; wise, elegant and on the brink of extinction. Why do these two stand out from the others? They have been at it for millions of years, before most of the other races even existed. The concept of two foes ceasing hostilities after millions of years of conflict is laughable... or is it? Perfect scenario for my concept!

While there are plenty of big nasty things to draw upon to have them fight together against. But I think the more pertinent quesiton is: On what common ground do these previous foes fight?

I like the idea of life being the central theme here. Life is such a brief and often violent thing, which some of us are forced to fight for every day. How about two foes fighting over life, whether that be to destroy, create or preserve it. Something big and nasty comes along and threatens both opponent's goals. Suddenly they find themselves postponing their previous conflict in light of the greater threat.

This creates intrigue in itself. Are these two truly stronger united than divided? Can they overcome such a foe? And most important of all. What happens if they DO defeat the big nasty? Do they continue to fight, or do they reconcile? Going back to my previous thoughts on this, I like the idea of a grudging admittance of respect, and a parting of ways. This leaves us with no illusions of the opponent's supposed 'friendship', but also gives us that traiterous seed of hope: perhaps these two people can come together in peace.