Sounds like the signs were there - "he has only ever fought when another dog jumps on him" seems like he's been building up to fighting for a while. He see HIS packs as a bit bigger now and he wants to cement a place as top dog.
My advice is you need to be the top dog. If dogs start a fight (no matter if they are mine or anyone elses) then I finish the fight using whatever means necessary. After you have them seperated yell and scream and hit sheds, kennels etc to scare the living crap out of the dogs that are fighting (ALL OF THEM) what this does is establish you as the pack leader and settler of all disputes....
T
Gotta disagree with you there it takes a pretty sorry dog to not defend itself if another jumps on and starts a fight with them. If be prone to cull the both of them if that was the case one for being to stupid to even defend itself the other to want to start crap and fight. I do agree that the pecking order has to be set and they have to know that none of that is tolerated first and foremost.
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