Will the neighbour's sit idly while you stomp their friend? I doubt it. You will get attacked from two sides instead. Nice theory though.
Diplomacy, my friend. There are often parties more willing to defend their friend than their friend is willing to defend them. Then there's timing: one of them can be busy with something else, or be blocked by a treaty that hasn't yet expired. Then there's diplomatic strategy.
Also, Chenier, I don't know about you but I don't think many people actually start a war just to completely devastate a few regions. When a realm attacks another realm, they will either do it to gain regions or for political regions not to cause genocide to depopulate a chunk of a map.
Had the daimons not interfered and resumed their invasive ways, Nicolas Chénier would have successfully genocided a good chunk of Beluaterra.
Genocide starts to be the only viable strategy, once your noble count is too low to justify colonies.
