Sure, you can negotiate for resources. You can try to play nice and share if it's not a single artifact of great power or the services of a master of the arcane. But other forms of constraints - time, orders from superiors, personal grievances - can make typically peaceful approaches just as intense as all-out war.
Does your game need to pit two factions against each other, but you're still not settled on why they should be fighting? Here's a few suggestions:
- Both sides claim an artifact (however powerful or mundane as you want) has religious or historical significance to their people.
- One side holds a monopoly on a certain art, substance or technology that has the potential to dramatically alter the other's quality of life.
- Both sides want the political, religious or social clout to accomplish some goal, and must compete for support from a number of prominent figures.
- Both sides have a common foe, and their leaders have agreed to work together, but neither side approves of the distribution of power, food, knowledge or shelter, feeling the other side "has it better."