How to defend less advanced species from invasion?

I have been reading several stories about this topic. Where a less advanced world could be under threat from a hostile conquest driven interstellar empire. If you are a similarly advanced civilisation with the desire to defend less advanced civilisations how would you go about it? I have seen contacting them and uplifting them so they can defend themselves. I've seen secretly defending them from space without them knowing.

What would you do if you were an advanced civilisation and a less advanced civilisation was under threat?