With any PoD (because I'm honestly not that sure what would be required for this to happen), make dogs a large part of warfare on par with cavalry or archery. what part would dogs play on a battlefield? I imagine they would be relentlessly effective against lightly-armed/lightly-armored opponents, but falter against cavalry and heavy infantry. This would make them the perfect anti-skirmisher weapon, especially if a people like the romans adopt them against tribal societies - even a germanic breserker can't shake off three rabid dogs.
Also, how would such a division of the army be called? I'm thinking either doggery or houndry.