So, say we have a country divided into relatively independent city-states. What reason might the ruler of city-state A have for wanting to control neighboring city-state B? (Said reason may or may not have to do with a secret alliance that city-state A is forming with people outside the country--I haven't decided on that point yet.) Note that in this scenario city-state A is the enemy (though the motives should be understandable) and city-state B is the "good guy".
What I've thought of so far:
-the lands of city-state B contain some sort of natural resource (precious metals? building materials?) that would increase city-state A's wealth/prestige/something-or-other
-someone important in city-state B did something to really piss off the ruler of city-state A (though I haven't yet thought of what might be such a big deal that it'd cause outright war, while keeping city-state B on the "good" side of the equation)
-city-state B lies between city-state A and something else they want access to
Any thoughts? Spins on those ideas? Let me pick your brains. :D