In the middle ages this basically amounted to open season on whomever it was that started the ruckus, which is how I'm treating it.
True enough I guess, but then again in the middle ages (and, well, today still :) ) leaders considered alliances as convenient bits of paper and little else, especially when they had a large amount of soldiers and mercs sitting around twiddling their thumbs and ravaging the countryside to pass the time.

Also, "open season" is probably not accurate, as many "wars" and battles consisted in one side muscling in, the other bringing their own lads to show they were willing to fight, then they'd negociate and everyone goes home. Understandable, especially considering the "good" season for war is summer, when the peasants are sorely needed back home to tend to whatever mud puddle they called a field back then.