world enough and time
A former superpower fragments, dissolving into two factions in a long-term civil war – each side battling for supremacy over arcane philosophical standpoints which matter less and less as time goes on to all except the active participants.
Resources are a background concern: adverse weather patterns, aquifers and soil layers showing themselves to be most definitely finite. Consequently, horizons have shrunk – rather than being part of a particular larger faction or great nation, things now focus on the local neighborhood level and its defense against outside exploitation or incursion. The conflict is only relevant when it physically churns through and wrecks things.
As resources dwindle and supply lines melt away, the armies are forced lower down the tech ladder: fancy weaponry cannot be repaired. Dirigibles, trebuchets and their ilk are primary forms – and trebuchet teams are not at their best when faced with both retreat through boggy terrain, and hostile bayou denizens (human or otherwise) as well.