Intervening is NOT always a good idea. My son has tried to break up fights twice where both parties turned on him and beat him up pretty bad. The second time he regained consciousness in the street where they had apparently shoved him into oncoming traffic. The driver that struck him stopped to help him.
My advice (to him) is NEVER try to break up a fight between two people you don't know. They probably already know each other, are enraged, and might welcome a common enemy.