If your dogs suddenly start fighting with one another, the reasons can vary.  Have any of your dogs not been neutered? Has one become sexually mature?  Is there plenty of space for both of them to share?   Have you let them get away with bad behavior in the past? Do they get little exercise or have too few dog toys?  Any or all of these things can cause fighting.

To correct the problem, exert your authority and feed the more aggressive dog last.  Make him walk behind you and your other dog. If he does show aggression, give him an immediate correction such as a loud noise. This tells him that fighting will not be tolerated. Always reward him for peaceful behavior. This will motivate him to repeat the good behavior. If the problem continues, consult your veterinarian.