Like with most things, I think the right move for an adult is to help the child solve the problem themselves, rather than do it for them. As other posts have mentioned, learning when and how to apply violence is a critical skill. We (particularly children) do not live in a post-violence society. Children should of course not feel abandoned by those closest to them, but being overprotective can have its own negative consequences for development.

It's clearly a fine line to walk, but if you're being bullied then what you need is to learn how to address/discourage that behaviour - not an intervention. Otherwise all you're doing is deferring the learning experience until next time. Past a certain window it's very hard to learn this skill, and you can be stuck with a helpless mentality for your whole life.