If sustaining a side requires waging war, then Erfworld must have some particularly weird mechanic that supports that.

In the real world, war is a net destructive process - the winner may end up with more resources, but only via transferring resources from the loser; the total resources of both sides combined is decreased by the war. If Erfworld matched reality in that regard, then every war would decrease the resources available in the world, and something would need to counterbalance that or the world would inevitably slide to total resource starvation. But if such a counterbalancing resource creation mechanism existed, then a side would be able to support itself using that with no need for war.

One possible explanation I thought of is forced overpopulation - cities are always working on popping something and "nothing" isn't an available option for a ruler to choose, so a non-warring side will eventually accumulate more units than it's able to support. This could be handled by croaking, disbanding, or harvesting for provisions any units above a certain amount, though, and if the solution were that simple Parson would have already found it. He definitely thought of it, he goes into significant detail on it in this Klog, and if overpopulation were the problem then this would be the entire solution, not just "one more piece of the puzzle".

So, somehow war in Erfworld apparently creates more resources than it destroys.