It is 2150, and there are two major human settlements on Mars and a dozen smaller branching-colonies. The colonies, Genesis and Metropolis, founded by Space X and Mars One respectively, both have a population of two million people. Genesis is a direct democracy, while Metropolis is an indirect democracy.
Both have around six settlements that have branched out from the original two for various reasons, and ideologically and politically they are part of the original two. Several other, newer corporations, seeing the opportunity in Mars, have started planning preliminary missions to the Red Planet. One has decided to start a new colony outside of the legal territory of the others. The rest only intend to transport people from an overpopulated Earth to the new colonies for a low (10,000 USD in modern money) fee.
There have been no serious efforts to terraform Mars as of yet, though the executives of Genesis have discussed building "terraforming stations". The general climate between the two Martian colonies is "friendly" and they have yet to have a major dispute. Culturally they are melting pots, with settlers of Chinese, Russian, and American descent. They are both secular states, with a minority of people following any single faith. Most believe in a "god" of some sort, but don't associate with a specific religion.
They both have limited numbers of firearms on hand. Importation of weapons has always been minimal, due to the dangers of projectile weaponry. Only enough weaponry to maintain order had been brought to Mars. No high explosives, only about 100 or so automatic rifles, and about 500 handguns for each. Otherwise, they have about 1500 Tasers, and 2000 nightsticks. In addition, they have some 500 rovers each, and some 1500 automated security droids, which are on average twice as strong as an adult male, twice as fast, and possess built-in stun guns and pepper spray.
There is no unobtainium on Mars, just the regular natural resources that you would expect.
What I want
I want a war. Ideologically, they are similar. Culturally, they are similar. They are both secular states. There is only minimal weaponry on-world. They are on good terms with each other.
I seem to have written myself into a hole: I cannot think of a reason for them to fight each other. My first thought was resources, except there is literally an entire planet of resources for them to mine and extract. There is water ice, natural gas (frozen in the poles), and minerals aplenty; and they both have plenty of room to expand. I need a rapid degradation of relations between the two to the point that they are ready and willing to take up arms in the first Martian World War. (Technically, it would be a World War, right?)