'Neighbours... hah. People'd live for ages side by side, nodding at one another amicably on their way to work, and then some trivial thing would happen and someone would be having a garden fork removed from their ear.' And when the neighbours in question are the proud empires of Klatch and Ankh-Morpork, those are going to be some pretty large garden tools indeed.
Of course, no-one would dream of starting a war without a perfectly good reason...such as a 'strategic' piece of old rock in the middle of nowhere. It is after all every citizen's right to bear arms to defend what they consider to be their own. Even if it isn't.
And even if they don't have much in the way of actual weaponry. As two armies march, Commander Vimes of Ankh-Morpork City Watch faces unpleasant foes who are out to get him... and that's just the people on his side.
The enemy might be even worse. Discworld goes to war, with armies of sardines, warriors, fishermen, squid and at least one very camp follower.