How do you break up a dog fight?
From a distance. Depending on how intense the combatants are, a water hose supplying a cold shower could put a damper on their hostility.
Trying to physically remove a combative dog from a fight may leave the other dog free to attack more fiercely, and there's nothing to prevent one of the dogs from turning to attack you. This method is not advisable.
Actually, the best suggestion of all is to be proactive instead of reactive. In other words, don't let a fight start in the first place. At the earliest sign of a dog displaying serious aggressive warning signs, get your dog away from the situation. Severe, stiff body posture, growling, snarling and snapping are signs that a dog means business. Don't believe the owner who ignores the problem by saying: "Well, Sparky always acts that way, but it doesn't mean anything."