The situation is thus:
Playing 1.16 Ironman, I wish to attack and dismember Bavaria, which is, of course, part of the HRE. Bavaria is about its historical size, and has a few small allies.
I am a much-expanded Italy and I can easily take on Bavaria and the minors by myself (famous last words?). My allies are über-Bohemia and my march, well-fed Ansbach.
Now, Bavaria and the HRE Emperor (Austria) loathe each other. Austria has a -300 opinion of Bavaria. Bavaria is Protestant, Austria is Reformed (!). I am Protestant myself.
(The Commonwealth, France and Castile are the only major Catholics left, BTW)
My question is this: a) if I declare war on Bavaria, will Austria join the war regardless of the fact that the little box has a red cross next to it? And (b) if she does enter, I assume she will call in her allies, which include the dreaded blue blob, as well as half-a-dozen countries in the Empire?