It's not in every household that dogs live in harmony. Dogs who lack socialization skills or aren't corrected during their young age when they become aggressive can turn dangerous.
A couple of days ago, my shih tzu decided to pick a fight with a bigger dog. The bigger dog was nice enough to put up with him but eventually the constant growling and biting from the shih tzu got to him.
I sprayed them with a water hose but the shih tzu still came after the bigger dog who already backed down. You'd imagine how that turned out to be.
Armed with nothing else and without anyone I could have asked help from, I grabbed each dog on the back of their neck, separated them and had them lie down on their side until their body started relaxing. (It's important to be cautious-but not hesitant- in doing this because there's a big possibility that you might get bitten yourself.)