Why bother with taking the poor and underdeveloped lands of factions like Gaul, Germania, Illyria, Thrace, Sarmatians etc etc? I´ve found that the AI is very prune on accepting the protectorate status initially and grows more and more reluctant to do so the more land you take from them. Theese faction are of no threat to you and will never be. Just beat them in a couple of battles. If they don´t accept a protect. or a ceasefire after you´ve beaten their armies just leave them alone and do nothing for a while and... shazaam a diplo will turn up offering you a ceasefire and the chance of getting the to accept to become a protect. is very high.
Personally I don´t respond emotionally when beeing attacked. I´ve just one principle though. If an ally betrays me I always go on a punative expedition. I stick with my "agenda" - the strategic plan I decided upon before playing.