While it may be crude and childlike, I begin all projects with a map. I suppose I work backward. I draw the coastline, make the mountains follow the coast, the water (rivers) fall from mountain to ocean, occasionally pooling in lakes and land-bound seas. Trees follow the water. People build along bodies of water.
National boundaries follow natural features. All together you have a picture of distinct realms, with distinct geographical features.
Human society and culture is an adaptive trait formed to help us deal with environment and social pressures. So once you have a map of where people live and what resources each of your realms have, you can start to figure out who gets along and who doesn't.
The creation of who "we" are and who "they" are is intrinsic to humans. We derive some of our identity (for better or worse) from establishing what we are not. "We are capitalistic," you may've heard in the 50's, or last week-- "and thus not communist."
Same premise goes for fictional communities. The wealthy countries and those hurting for resources will be the most likely to head to war. However, the circumstances are completely different.
Then, of course, there is magic that will be tossed into the mix. I like to attach magic to a main character or strong supporting character and treat magic like a resource: it makes the magic-user political, lands them in a leadership role because they have a control of a commodity that others don't.
Of course, how important magic is in a world (how much prestige and thus authority your magical character commands) is determined by its frequency in the population, how critical it is to the commercial, governing, and political realities of the world.
You could also chose a magic that cannot cause harm. Those ones are interesting because the magical person is thrown into a whole bunch of conflicts you can't touch with more weaponized magics. So the moral, social, and psychological issues that you can explore in the character and plot development explode with potential ;-)
Tomorrow's post will be about resources and how they set up politics, culture, and such. Then on Thursday I'll take it back to the individual. after the whole world is built up, how can I reduce it to what-is-seen by my character?