You brought home a second dog for company, and now you are lying awake listening to a growl travel through the baby gate. The Second Dog Was a Mistake is for the owner in that moment: the one splitting the house into zones, feeding in separate rooms, and wondering whether they have failed one dog, both dogs, or themselves. This book does not promise that your dogs will become friends, and it does not ask you to manage aggression on your own. It walks through what is actually happening between the two dogs, why a sudden change in behaviour means a veterinary appointment before a training plan, how the layout and routine of a home create or defuse conflict, and how to find qualified force-free help without pretending it is cheap. From there it lays out an honest decision framework for coexistence, permanent separation, or rehoming, and treats every one of those paths without shame. Written in plain, steady prose for readers with no yard, no second adult, and no spare money, it is a companion for the hardest part of a multi-dog household: deciding what kindest looks like, and then living with the choice.