I intervene! My dogs are my responsibility and its my duty to protect them. If they were attacked, I wouldn't think twice! Equally, if mine attacked (and my roar didn't quell them!), I'd act to protect the victim, by whatever means necessary.
Pick the aggressor up by the back legs and step backwards. This is definitely the most effective and least perilous way to break a fight. The thing is, you often need two switched on people to manage it, because once the aggressor is disabled, the other dog can sometimes try to re-engage, if not restrained.
If to hand, the sound of a fire extinguisher is extremely effective, as is a bucket of icy water.
Using your feet is dangerous, however stomping down is more effective than kicking. We generally have a stronger stomp!
Obviously, avoiding an attack is key and preempting a possible attack, blocking where possible is ideal. Sometimes even a strong NO. GO AWAY. with yourself between your dog and the aggressor can deter an incoming dog (although don't get in the middle if they're inches apart!). Sadly, it's not always effective or possible.